Administrator theymos will probably give you a second chance if you refuse from signature.
Contact theymos and see if he can implement a specific signature ban for you instead and reinstate your account. This is something he said he might implement a while ago (and I'm sure he could do individual ones now in certain cases). A signature ban would probably be more appropriate in certain situations and especially to those who are going to contribute in other ways whilst removing the possibility of earning here (which is probably a significant punishment in itself).
Maybe signatures of spammers/plagiarisers should be removed indefinitely or removed until they've achieved x amount of merit then they're reinstated. Most users probably won't bother and just abandon the account but for those who aren't here just to earn and are willing to contribute maybe that would be a sufficient penance for their 'crime'.