No there is nothing you can do. You sent irreversible currency to a con-man. It is gone. A hard lesson to learn but always make sure you are sending money to the right person.
so fucking gutted. 3.4 BTC is lot and its a new business start up as well. I obviously thought it was the right person. I'm surprised someone was able to register a username that similar and contact bidders of the advertising auction unnoticed - This shit needs policing