I partially disagree with you. I've seen scammer (who have high rank and have some negative tags) add scam website on his signature and write non-spam post on board that doesn't show trust number. If those scammer are banned and forced to make new account, at least they can't use big-sized signature or attach image without additional effort (such as obtaining merit or spending money on copper member).
Well, that scammer's reputation is already ruined and with such reputation even applying a signature of scam website won't help him/her much. Banning accounts of such scammers is going to put so much burden on the shoulders of the admins/moderators and even with bans they can still create new accounts and find ways to continue their old habits. Although, you're also right that they may need to buy copper membership to embed images in the posts but still that won't help the forum in any positive way.