this will most likely not happen because other rule-breakers without sig ad will get harsher punishment. also I don't see how this is better than temp-ban in terms of teaching them a lesson.
Instead of giving them a temporary ban, you remove their signature permanently. The problem resolves itself, their account becomes effectively useless in regards to signature campaigns. This seems much better than the current way of handling things.
ban is better for spammers, heavier punishment is needed and if they dont want their account to be permanently banned then they should improve their posting quality.
Usually a lot of them don't. They keep spamming and then switch onto their next account if the current one gets banned.
If they keep spamming, how is removing the signature a solution though? If the problem is spam the solution is to stop the spam. The incentive
might be the signature but its hard to tell in advance. Maybe it can be an additional option to remove the signature. E.g. short period ban (3 days), followed by a slightly longer (7 days) removal of the signature. That way you can see if they just wait the 7 days or actually want to contribute w/o getting paid for it.