Hang on, are you referring to our wiki, or to Sin Stalker's CoH Comic Index wiki?
If it's the latter, I'll have to look into it, but it might be a while since I've been working on a few other projects. I can tell you that on the Paragon Wiki, the account creation mechanism within MediaWiki has been disabled and all accounts must be created via the Titan Key page. I think that saves us from most wiki spam, since the spambots don't know how to register Titan Accounts and no one has (thankfully!) written a Titan spambot yet.
My honest suggestion would be to create your own account creation and authentication mechanism, something that the spambots won't easily know how to get around. Other than that, there are ways within MediaWiki to deny write permission to all but some accounts. It might not be ideal, but you might want to consider doing something like requiring all new accounts to be validated by an administrator before allowing them to edit the wiki. I can't give you the details on how that's done because I'm at work and can't look into it right now, but it might be a good starting point.