You can use Batch update in the ACP for this.
You'll have to identify the users yourself that violate any sig guidelines, the process for this isn't automatic.
Easiest thing to do, which I did on my site and was also done here on XenForo when these permissions were introduced is to tell your users that you'll be purging all signatures in a few days time. Then remove the lot using the batch update in the ACP. After that everyone can redo them and in the process you'll hopefully get some nice new signatures.