I really think DT system should be used to prevent people from scamming (or make it a little more difficult at least).

If we leave negative trust to shitposters I think people can end up ignoring the feedback left to scammers/untrustworthy users.
While it would be fair to tag shitposters somehow and stop them, it's certainly not fair to put both them and scammers in the same group.
Scamming or trying to scam is definitely worst than spamming.

Maybe leaving a neutral to users with extreme poor post quality would help? I guess signature managers would have to place some rules about those neutrals besides the current negative ones.