Hmm, never knew of that one before. I thought that sending and receiving a PM from newbies are the only cases wherein you will see a big red warning, or am I missing something?
Put a neutral on the account's trust rating. In that way, people will (or at least they will) view the profile first before making a deal with the person.
Nope, PM's from newbies are the only PM's that carry the red warnings with them.
As for leaving neutral rates, I think that a negative trust on the account would be more effective. It's pretty clear that the impostor is trying to scam users, and I think that deserves a red trust. Like hilariousandco and Quickseller mentioned, lots of people wouldn't bother checking out a user's trust before sending coins.