a sledge hammer can definitely put that machine out of commission. A
pickaxe would be even better.
But, honestly, why destroy the whole computer? If you don't want it, give it away. If you're concerned about data on the hard drive, just destroy the hard drive. A local computer users group here has recycling drives. They destroy unwanted, i.e. too small capacity to salvage, hard drives with a smallish pickaxe. If the drives are good to keep and put in a refurbished machine, they do a thorough data wipe before putting them in any working computer.