I would suggest leaving neutral feedback starting with "Spammer" or whatever word we agree on, and ask signature managers to disallow users with that trust.
I could get behind this solution if signature campaign managers could agree on it. Doing this would also semi-invalidate SMAS, which wouldn't necessarily be a bad thing.
I would leave negative feedback to spammers only if they have untrustworthy behavior such as constantly copying and pasting content
There'd be little point in this; it would be more effective to report the account and get it banned. Should the account not be banned, perhaps this would be a good way to continue.