That's a nice modification. I had figured 10 (or maybe more) posts would be a good minimum - everyone has had posts delete for a variety of reasons, but it would be fairly unlikely that you would have 10 deleted in a short space of time unless you were a serial spammer - but I agree there might still be some false positives.
I do think there needs to be some form of punishment though. Deleting a hundred posts makes no difference when three hundred more spring up in their place.
I mean to have read that there is a punishment - mods are regularly deleting/banning lots of "spam accounts", above all, bots. But it seems to require manual action, which means a lot of work. So a technical solution indicating "serial spammers" would be certainly nice. (I'm only speculating; I don't know SMF in detail. I once had a small Drupal forum and there were no such "helpers" at that time, but surely there are extensions for that task.)
Doesn't a thread get hidden if you ignore the person who has started it?
I just tried that with a typical megathread on the Speculation forum - and no, it didn't work, the thread continues to appear. The idea is good though - because if a spammer you have on your ignore list creates several threads, they would be all hidden by default.