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.

Helping to keep the SMAS list up to date and asking signature managers to use it can also help.

I would leave negative feedback to spammers only if they have untrustworthy behavior such as constantly copying and pasting content, as that can be seen as trying to defraud the signature campaign. But absolutely not to poor quality posters. Negative trust means "scammer" or "very untrustworthy"