Nightmare. Never use unencrypted wireless communications. Yes, a MAC filter prevents anyone from connecting to the internet through your router, but it doesn't prevent anyone from listening to your unencrypted network traffic, and pulling out of that whatever you send in plain text. Very bad idea. You should either use encrypted wireless or hard cables.
Do not compromise in this area; wireless sniffing is a growth industry and thieves want your unencrypted personal data.