I am a peaceful fellow, but come on. Holding his shoulder? #1, that's a great way to get stabbed or maced via the other hand, #2, it's pretty easy to shake someone's hand off their shoulder, grab the bike and vanish into the night, and #3, the guy pretty much gave his consent to be choked when he tried to steal someone's bike.
Awesome story, OP.
I think if the OP is as manly as he seems to fancy himself to be, he could put down a pretty good grip on the guys shoulder and hang onto him long enough that other people would get there to help. If he's in front of a bar and isht starts going down people are going to come see whats up mad fast.
P.S. I'm curious as to the bike that was the victim of the almost-theft, pictures?