I'd say two to the chest and if that won't do it one to the head, if I even think the sack of shit is going for a weapon but the sad truth is it's just big talk. If I do catch a bike thief it'll be during working or drinking hours and the only weapon on me will be my razor sharp whit.
clearly should have just quickly kicked the guy in his unguarded belly. the natural response to that is to double over due to the intense pain. at which point you score extra points by giving the guy a kick in the nose/mouth/eye. hopefully then he is so hurt that he falls over, and this gives you the perfect opportunity to step on his neck, ribs, head, anywhere really. preferably get him in the nuts. over and over. until you feel sorry for him and become disgusted with yourself for being so violent towards another human being.
I'm reminded of something one of my bar buddies once said. "Us old guys are don't like to fight, we're to tired for that, that's why we carry guns." In all seriousness I really hate the notion of doing violence to another human being but if I have to and it's a bike thief or home invader I imagine I won't feel quite as bad about it. Come to think about it if someone tries to steel a bike off the rack at Darwin's they're lucky if I'm the one that catches them. Some of the regulars are really nice guy's until you piss them off...........