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.
I'm talking about signature spammers in particular. In 99% of the cases the incentive is the paid signature. Now what you're suggesting seems acceptable to me. However, you're a bit soft. I'd go with a 3d ban followed by 30d removal of signature. It should not only be a 'test' to see whether they are going to continue posting in the period, but rather a punishment as well.