It is an interesting problem I've never asked myself before. I was looking for some information, and I found something, that may be useful
What I think you should do is to establish a limit for the number of processes a "normal" user can own (perhaps a good idea is also limiting the number of files that a process can open, to avoid more problems in the future).
How can you achieve this? Several ways:
1) check "man limits". With a file called "/etc/limits" you can easily do it.
2) Maybe another possibility:
(however, it could be a bit old, I have no idea about it
3) You could do it by hand: a couple of scripts... it should be possible
I hope it helps (EDIT>) you to give a "funny" surprise to this "funny user".