I ask you to give me hints about how can I locate that cracker
You won't have much a chance there; if he's a serious spammer (not cracker, by the way), he won't be using his own computer, but rather have taken over some poor soul's machine (usually without their knowledge). Don't waste your time on this.
how to avoid this vandalism in the future.
http://www.mediawiki.org/wiki/Manual:Combating_spam
HTH.
-- F.