The spam problem has been largely dealt with (>500,000 messages deleted). Apologies for this, if only I'd been more aware of the scale of the problem it would have been dealt with sooner (and I wouldn't have had to spend three days cleaning up).
Mollom is installed and is currently rejecting >50,000 spams per day. If anything gets through, the best recourse is:
- Click the user to visit their user page
- Click edit
- Down the bottom, click Cancel Account
- Choose to "Delete the account and its content"
- Choose to report as "Spam, unsolicited advertising" - this will improve Mollom's moderation
- Cancel account
This marks the whole user and all content that they have posted as spam, which will hopefully improve future automatic moderation. I have been doing this for a few days and it appears to be effective - hopefully effective enough in the long term to not have to resort to more drastic measures.