He was a star defensive back for the Hornets and he's two minutes younger than me. so physical threats dont really work.

You don't have to be bigger, ever seen the New Guy ? just give him the crazy eyes! ;)

im half serious, maybe ask nicely first try explaining how annoying it is and ask him nicely to stop, if that doesn't work next time he does it completely fly off the handle, show him your willing to kick scratch and punch to protect your pc, he may be bigger than you but if you could cause him alot of pain everytime he touches your computer it may work....

BTW why does he use your computer instead of his own ? is it because he knows he'd mess his pc up ? if so go have fun on his pc, download every dodgy file you can get your hands and click every little pop up involving porn (i would recomend installing zango as well, sure to annoy)

If nothing else works how about your parents ? will they help sort out this situation or will they simply tell you to get along ?