Is the message different(unique) for every account?
If I contact Poloniex can they block his Xem wallet?
How to prove that it was not me sending coins?
Is there ANYTHING that can be done in this case?
Yes but the chances are (unless he's really stupid) he would give a fake name and address and would only have basic verification on the account. Then they'd sell and transfer the bitcoin out. Also you would have to prove that the funds were stolen rather than willingly transferred to that person (impossible). I don't know but I think you're probably out of luck. Sorry
