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.
At that point, why use the trust system at all.
Neutral feedback makes it hard to spot those users (eg think about a campaign with 100 users, a manager would have to parse 100 trust pages to spot such tags, as they dont show up like negative trust does).
The reason some use negative feedback to tag spammers/shitposters is because it stands out. It's easy to spot.
Helping to keep the SMAS list up to date and asking signature managers to use it can also help.
If we already achieve consensus among campaigns on how to deal with this problem, leave out the trust system.
We dont necessarily have to use SMAS, but an apporach similar to it would be more easy to manage.
A public collective list that is easy to request and check.
If you want signature campaigns to use it, make it so they can do that without creating a huge extra effort.
This could work out, but what it needs is widespread support from all campaigns.
The way SMAS is right now, it's impact is limited. Utilizing the trust system has a way bigger impact as it affects campaigns that don't directly support the fight against spam.
(Note that I, currently, do not leave negative feedback for spamming/shitposting. My apprach to this problem was initializing SMAS, if you want to contribute to that approach, feel free to contact me.)