The evidence is more than good enough, so red tags and/or red flags can be left on the profiles by you or any member.
Correction: "red flags" (I assume you mean type 2 or 3) should only be created by actual victims of the scammer.
See the Flag creation page:
[type 1] Due to various concrete red flags, I believe that anyone dealing with this user has a high risk of losing money. (This flag will only be shown to guests/newbies.)
[type 2] This user violated a casual or implied agreement with me, resulting in damages.
[type 3] This user violated a written contract with me, resulting in damages.
Which is unfortunately evadable via alternate accounts. Once a scammer is given a red flag, there is no consequence to repeated account creation as a means of trust evasion. Of course, since trust is subjective, all this means is that if the scammer is more persistent than the victims (a likely scenario) apropos to scamming, then you can expect an efficient scammer to have a yellow flag at worst, and no flag at all at best.