Vandals are the bane of every wiki's existence, those little buggers always keep annoying the heck out of people! They usually destroy wikis like by blanking a page, filling it with garbage etc.
Known Miraheze vandals
This is the easiest, you just delete everything and click "save", note that it won't save you from the banhammer so be careful, loser
Filling it with random junk
Just replace the article with random key mashes and ur good to go
How spot them.
know it when I see it.
Does it really need to be defined?
It's arguable that most (identified) vandalism has consisted of really quite obvious cases. Hence, Wikipedia doesn't need to define an "official" policy on what constitutes vandalism at all. We can use the rule of thumb, "When a reasonable person might be in doubt as to whether something is vandalism, it would be polite not to call it vandalism."