Thanks everyone for jumping in here! I turned off account creation again (for now), blocked a bunch of accounts, and mass deleted the pages they created. Also restored the Main Page (thanks for pointing to it!). I'll continue later, but I am a little busy at work atm / today...
We have faced a lot of wiki spam - we have Captcha on account creation, some custom anti-spam code, no anonymous edits. But eventually we just fell too far behind. Like MultiplyByZero said, I think these may be a mix of bots and real people. Surely we could get to a decent setup with upgrading MediaWiki and finding the right plugins, we just never made time (yet).
And yeah, I'd love to discuss getting a new admin or two on board! We haven't added anybody new in a while and indeed all the current ones are sort of retired.
I have been through all the accounts of the 'Active users list' and Mass deleted and blocked the appropriate accounts. Seems that all New created pages are deleted at this moment. In that active users list still some spammers are listed, but they did more subtle things like minor edits and moving pages. Someone with a bit more knowlegde of wiki-stuff should go through their contributions and try to erase those.
Thanks for this colossal work. I've seen you deleting non stop for at least an hour.
Would you be able to revert some important pages as well? They are listed above this thread in reply to Chase-san.