wait till hes only with like 1 mate and go psycho at him with a baseball bat, aim for his ribs with it and get him on the floor then stamp on his head a few times. aim to break a few ribs and a bloody face. enough to show your not f**king around but not enough to seriously hurt him. only one mate because then people wont say oh my god you ganged up on him when he was on his own etc. and if youre lucky he wont try and back the guy up because he'll be so scared of you.