No. If someone is a scammer, or has scammed in the past, but for whatever reason does not have negative trust then it would be appropriate to leave such negative trust.
Then why not give a negative trust rating to every single person that has ever scammed in this forum? The truth is that you can't and you won't because you'll only search for a reason to and give them out with ease to the people that have spoken against you thereby
attracting your attention.
If I
come across, or am otherwise made aware of someone who has scammed then they will get negative trust from me.
If you are implying that I should go searching every single thread for scams, then that is ridiculous as I obviously have a limited amount of time/resources and doing so would not be logically feasible.