Well I have seen people go with 3 methods to this problem to solve it.
One method is to shut off the wifi and have computers there for everyone to use that has the browser set with allowed sites added to browser in which all other websites are disallowed. The computers are yours and the customers can use them to sit down and surf and leave after a period of time. However this model is an older model for offering internet access and can be time consuming to maintain the systems to constantly be asked to allow more and more websites to the allowed website list in which you would have access to adding or removing sites that others can surf to. USB ports and CD/DVD drives are hands off to customers so that they cant use a hacksaw device and bypass your rules for computer access by use of a USB stick or Disc that say boots to Live Linux and gives them all privileges as well as privilages to modify your hardware and other systems etc.
Second method has a heavy initial cost and its what the majority of the world is using today. Its to have a device in series between your wifi access point or wifi router and the modem connection to the internet. It acts as a traffic cop to allow or disallow websites. BUT this too also requires knowledge of how to go into it and add or remove sites from white list.
Third method is what most go with a web filtering service provider. In which I contacted WebTitan and had a chat with them today to discuss and get some info for you. I copy pasted the chat below. You can go with any web filtering provider you like, but search results showed they were on the top of the list so I contacted them and gathered some information guessing your setup to be lesser than 50 people connected at a time to get pricing info.
For the first 2 filtering methods ther is the black list or white list option. One method allows everything except for what has been added to the black list. The other blocks everything except for what has been added to a white list. They both have pros and cons.
The white list method is the best method as for there are likely less websites to have to allow than there are to disallow. The unknown is gone as to what they could be accessing as for it only allows access to websites that you have deemed ok and configured as such with the web filter device. The initial setup of this is costly and sort of time consuming as people are requesting you to allow access to websites and you may have to do a google search from your own computer to make sure that a website is ok before allowing it to the white list or if its something like a well known website you can just add it and know that its likely ok. Eventually with this model the requests from people to have specific sites allowed should slow.
The black list method allows all except for what is on the black list. There are so many bad websites and places to go that using this method wouldnt be good. You wouldnt know that a website or url path would need to be blocked until they have already done something illegal etc. Most people use black lists on local computers where say they want their kids to not have access to specific websites, well you can tell a browser to shut off access to a specific website or websites or use a wild card and shut off all websites that follow a naming convention rule.
For the filtering by 3rd party method my guess is that they already have lists made up so that the majority of what is bad or inappropriate for customer content in a internet hotspot is already blocked so this would likely be the easiest for you but the cost might be more than you anticipated for yearly service cost.
Here is the chat I had with Jamie of WebTitan below and website link here if you want to surf their site (
https://www.webtitan.com )
Info
at 11:09, Jan 16:
Thank you for choosing to chat with us. An agent will be with you shortly.
Info
at 11:11, Jan 16:
All agents are currently assisting others. Your estimated wait time is 0 minutes and 7 seconds. Thank you for your patience.
Info
at 11:11, Jan 16:
You are now chatting with Jamie.
Jamie
at 11:12, Jan 16:
Hi How can I help you
You
at 11:12, Jan 16:
Looking to set up a wifi hot spot and looking for a filtering service that will disallow people from surfing porn and downloading illegal content on our internet connection. Was interested in pricing for a small internet cafe and if there is any hardware that needs to be added or if I set up a proxy connection thru you to filter content?
Jamie
at 11:14, Jan 16:
we have two options for you, we have a cloud version where we host this on AWS cloud server and there would be no need for any onsite hardware. Or we have a on premise version in which you would install on hardware?
Jamie
at 11:15, Jan 16:
Do you have any specific requirements other than the basic content filtering?
You
at 11:17, Jan 16:
Well ease of maintaining it and price pretty much. If on onsite hardware, would it be a web filter device that you provide or I would take one of my computers and use it like a proxy/gateway for all traffic to go though using your software?
Jamie
at 11:20, Jan 16:
I would recommend the cloud version just the fact of an ease of set and low management necessary, people only go with our Gateway service if they have specific requirements as it's slightly more granular. We supply the software which you can install on any appliance HiperV etc.
Jamie
at 11:21, Jan 16:
Do you have a rough user count?
You
at 11:23, Jan 16:
well with the user count, its an open wifi hot spot in a cafe so there could be any number of people connected to the open wifi internet connection at any time as they stay to eat or drink coffee and surf on their own devices. So Im not sure of actual count other than that its likely to be less than 40 people at a time
You
at 11:23, Jan 16:
Do I select a user count and then your service would only allow that many people to be connected on my wifi or?
Jamie
at 11:24, Jan 16:
Yes exactly so we work in brackets so you would fall into our 50 User bracket. For 1 year = $810
Jamie
at 11:25, Jan 16:
Would you be free for a quick call now, I can give you a quick talk through our product and discuss pricing with you?
You
at 11:25, Jan 16:
ok... so what happens if a person with the 51st device trys to connect it just states that sorry all internet connections are currently in use or?
Jamie
at 11:27, Jan 16:
No it will let them access the internet, and take an average of a 5 day user count, If you are constantly over the 50 user bracket it will flag with our support team and you would have to upgrade. I suggest that the 50User count will be adequate for you needs
You
at 11:28, Jan 16:
Ok... sounds good. Actually is there a good number to call you at when times are less busy here to discuss further? Im currently multitasking on phone and internet chat 
Jamie
at 11:30, Jan 16:
Yes 813-304-2544
You
at 11:30, Jan 16:
Ok Thank You for your time. I will call later. Have a great day!
Lastly to add... going through a filtering service provider if anything illegal occurs its less on your shoulders and more the fault of the web filtering provider. They are generally really good at blocking content/sites and so this should put an end to the troubles your having. Its a lot of work to try to block content on your own. I was setting up a wifi hot spot for a prior business I worked at for their break room, and it was determined that because of sneaky redirection sites out there people were getting around the blocking to sites by URL redirects and so I shut off the wifi and placed 3 computers in the break room that were locked down to where they had no access to system other than mouse and keyboard and monitor and the computers were in a cabinet with lock and key. I had to go in remotely through remote access software and add to white list web sites they wanted to go to when they wanted to go to a website not already approved. We went this route because it was the easiest and cheapest method for a break room to have internet access. When we had open wifi I had a content control in place and someone used a URL redirect to get to an adult website on the companies internet connection and a female employee saw the content and complained and so just short of removing all internet access completely, we went this route of providing 3 computers that were locked down tight so people could get to some sites and mostly not others.
