That's what I was referring to with services only allowing one (or N) accounts per person. A spammer could use the signature multiple times, but would need new email providers for every single account.

And as an uncle (?) comment points out, there could possibly be blacklists of signatures of spammers shared between providers. That definitely would cause some new problems, of course, but who knows what the future may hold...

Oh I see. I hope it doesn't ever come to that!