congrats seth, you met your first phisher.
This isn't really a "scammer," these accounts are simply bots programmed to send you a message upon adding you. Normally they're automated to add recent trades from tf2outpost.
My advice is to defriend, and if their profile allows for comments, just say it's a phisher. As well you can:
I highly doubt SteamRep will have the time/ability to mark all of these hundreds of hijacked accounts, nor, like I said earlier, are they entirely "scammers"