They'd move uderground (as in to other forums or sites).
It's hard to tell the difference, for example, some users promote their sites with the same signature used for the signature campaigns. (Like lightlord from bitvest and 777coin - he looks like a regular signature campaign user unless you know he's the admin of that site, the yobit.net account will also wear their yobit signature and bitcointm's account wears their signature).
So that's a good thing for this forum, yes? But I doubt that if they have an opportunity to wear a signature on another forum they're not doing it--but that's another story. In either case, the argument that you're driving spammers away isn't much of an argument (no offense intended, sir). That's the point of this.