GG IMed me and told me what was going on. Right now, I disabled new registrations without admin approval. Guest posting was not activated - what was happening was whomever was creating accounts, posting, and deleting the account. His IP is banned, though that doesn’t usually stop a real asshole.
I’ll reactivate registrations in a day or two, hopefully until after the guy has moved on.
I know that I’m not really involved any more, but you guys can email me at any time at firstname.lastname@example.org if something like this comes up. I check my email about five hundred times a day.