like him this is full of morons
He's full of morons? There's your problem!
Serious answers:
a) You've already mentioned you're not particularly au fait with IT. If he's anything of a decent hacker, then unfortunately this means you have pretty much no chance in stopping him getting into your kit. Some previous good advice already, encrypt your computers using a decent length, not easy to guess password. 12 characters, mixed upper and lower case including numeric and special characters, something like "$jK0L)b7x6f)*". Don't write this down. Ever.
b) Again as already mentioned, this is unfortunately a problem of discipline. If you get the opportunity, frape him. If he leaves his PC logged in, get on there, delete his personal files (make backups first), abuse his friends on facebook, transfer all his money from his bank account into yours (ok maybe not that one), basically destroy his digital life (not in a way that can't be recovered). Let him see what it's like to have someone go through and violate your personal life like that, might teach him a lesson?
c) Give him some good and bad examples of what his "hacking" could lead to. Show him some of the sentences that people who have been caught have been given, such as the Lulzsec guys a few months ago:
http://www.bbc.co.uk/news/technology-22552753
Also show him some job descriptions (and more importantly salary levels) for security specialists. If he really is as good as you're making out, he could be earning megabucks
Finally, as already mentioned, he's probably not really as good as you think. It's relatively easy to "hack" into people you know's accounts and systems you're familiar with. Hell, I could get around anything my mum put on the family PC, and the majority of stuff on our school PCs when I was 14, and I'm not by any stretch of the imagination a "hacker" or security specialist (and certainly wasn't back then!!).
TL,DR:
- If he is as good as you're making out, and you're not techy, you're not going to be able to "out-tech" him, so it's a waste of time trying.
- The only real way you might be able to get through to him is to get him to realise the consequences of his actions, why he shouldn't be doing it, and how to use his skills constructively.
- He's probably not actually <that> good anyway, so don't worry about it.