Funny, Zacam and I were discussing this the other day. Five to six a day is hardly 'getting out of hand' IMO, but it is something that needs to be dealt with before it gets any worse.
I had time to look at this briefly this last week, and there isn't a heck of a lot out there from my quick search. It's kind of against the idea behind an open wiki, but at the same time, it is needed on todays 'net.
I seem to recall seeing a captcha plugin for mediawiki, but something that uses the stopforumspam database would be optimal I think.
I think I recall seeing a script designed for just that. It's not pretty, as in doesn't integrate with the special pages in mediawiki, but it may be just what we need.
Another idea was to tie the usernames on the wiki to the HLP forum and only registered users here could do any editing there, or some form of that. be it usernames or just passwords, but with the possibility of contributors not necessarily being registered on HLP (not to mention naming conflicts or mappings), it was put aside.