This boy needs to clean the latrines with a toothbrush for a few months
If you have a known company policy against using the computers for non-work related things, then you could fire him for violating that policy and shirking whatever duties he was supposed to be carrying out while he was posting. However, be careful if that policy has never been published and explained to employees or if it hasn't been used against anyone else who was using the computers for non-work stuff. If you have no other reason to fire him (such as poor job performance or insubordination of a type for which you have fired others) then you are inviting a lawsuit by firing him.