MEDIAWIKI was a poor choice and is used by SPAMMERS
Fragment of a discussion from Project:Support desk
I have the similar experience but every platform is targeted by spammers. Luckily each reputable platform has its own tools to combat those.
Mediawiki 1.18 comes with several extensions useful to combat spammers: ConfirmEdit Nuke RenameUser
In early versions one can install them manually
I also recommend User Merge and Delete CheckUser
HTH
