A solution could be to disable signatures for anybody at -4 red trust DT2.
This would undoubtedly lead to abuse where several members on the DT2 aren't really considered trustworthy themselves and have a few negatives from people on DT2. Anyway aren't most signature campaigns disallowing people with red trust from joining? So it should not be a problem.
Yeah, I don't think this would be a good idea and would lead to a lot of hysterical crying and possibly malicious abuse. Most scammers can't join signature campaigns anyway, but on the flip side there are campaigns that will accept them, but I don't think negative feedback should wholly disqualify you from campaigns (but this should be decided on a case by case basis).