Generally, in most cases excluding them from your trust list would be the best option if it's the simple matter of "I don't trust this person".
No.
Your trust list does not say anything about how trustworthy a person is.Your trust list should
include people who "give good trust", i.e. whose trust ratings are useful for yourself and possibly others.
You could even add a known Scammer to your trust list if, and only if this person is careful about giving meaningful trust ratings (even though I would advise against it).
Your trust list should explicitly
exclude people whose positive trust ratings have been given to untrustworthy people or whose negative ratings have been given as "retaliation" or whatever.
However, I would agree that sometimes a negative to warn others might be the better option. Entirely subjective of course, but if you believe someone is gearing up to scam or has shown scammy behaviour without scamming then leaving negative feedback could probably be justified to a certain extent.
That's the way I use my trust list. I also offer to delete negative ratings once I see "betterment" or a reasonable explanation for someones behavior. I even sometimes delete "old" trust ratings when I no longer deem them appropriate, maybe because the user in question has "changed" in my eyes. Obviously, I can only do the latter for users I know quite well from our local community.