Besides plagiarism, there are several other reasons that can also make users get negative tags, namely trolling. Yes, trolling is one of those things forum users shouldn't do if they don't want to get negative tags.
Plagiarism is a different case from trolling or even shilling scam sites.
The offender of the plagiarism once proven as guilty has a heavy punishment like a permanent ban, not a red tag. I don't usually support red-tagging about trolling because that is not appropriate on my part, maybe a neutral tag would be the right one.
For the red tag, I think if the offender is proven and guilty as a scammer or it could be harm other users, it would be best to put a red tag on the profile to warn other members or much better create a flag that would be suitable on this case. Any financial matters that you feel that can harm others will deserve to be red-tagged including cheating and abuse.
Read the
LoyceV's Beginners guide to correct use of the Trust system for the proper use of Trust System.