To be honest, removal of their signature to signature spammers might as well be a permanent ban and most will probably just give up immediately if they can't earn here...
What about spammers who are paid for posting? They are often stealing posts from other users to bump topics, I doubt that signature ban will stop them.
BTW I saw an identical message about temporary signature ban in some of users profiles:
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Banned from displaying signatures until August 02, 2019, 07:05:09 PM