[Julia_Gillard_the_Honest]

Oh how I have been there (except not with impersonators; just someone who came and complained on my reputation page when I said that I had found a better price for a hat and found his Get-lost! response unnecessarily rude). Basically, treat it calmly and professionally. Perhaps even give a response saying that it was the impersonator and even a SteamRep link. People will read it and you can continue buying/selling just fine with those people.