I'm still working on the problem, but I wasn't able to allocate enough time to finish my objectives this weekend so it's going to spill into the work week.
My main objective is to make it easier for mods to nuke spam accounts. We used to have this feature, but I'm reimplementing the "Nuke Account" button that lets us (reversibly) annihilate accounts and all their topics/posts/PMs in one click.
My other objective is to slow down the rate at which spammers can pollute the forum. I'll start with a Recaptcha per topic/post, but ideally new (untrusted) accounts will have a rate-limit that gets relaxed as they become trusted.
That combo makes it annoying for spammers to spam while making it trivial for mods to undo the spammers' work, with little impact on existing members.
As I mentioned earlier, there are more robust ideas I've been considering, but they take much more dev work and can be built on top of the two approaches above.
It's mostly done, but I'm currently testing my progress so far.