The eridu-generated threads show that the direct reply toll doesn't seem to work, or at least it didn't in this case. I still don't like the idea of the indiscriminate whole-thread toll, but I'm no longer expecting the current alternative to be effective.

I've thought of another option: maybe prohibit a user from posting anywhere in a subthread under any significantly-downvoted comments of their own? This is another feature of all bad threads that could be used to automatically recognize them: the user in a failure mode keeps coming back to the same thread, so if this single user is prohibited from doing so, this seems to be sufficient.