What could be done instead is to give moderators an automatic "hint": if (e.g.) 10 posts of a single user were deleted, a checkbox could appear which would ban the user (or delete his signature), but the final decision belongs to the mod.
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.