Excessive violence is bad. Was the OP excessive in his use of force? Probably not but these things are difficult to determine and really depend on a lot of different variables. A few weekends ago I bought a rusty old bike to fix up for my girlfriend. I had just brought it home and locked it to a post in front of my apartment. My girlfriend and I went around the corner of the building to smoke a cigarette and while doing so I noticed the front wheel (the only part visible from where we stood) of the bike moving back and forth slowly. I walked over and found an old man had already undone the quick release and was trying to take the seat and seat post out of the frame. I responded pretty much as frankstoneline suggests and asked the guy what he was doing in very polite Korean. He looked embarrassed, backed off slowly and told me that my bicycle was "really nice".
Now I could have grabbed the guy by the throat and made a big deal about it but that would have been stupid. The Korean police do not take kindly to foreigners roughing up old Korean men. Aside from that, I was looking at potentially losing the crappy seat and post off of a bicycle that only cost about 30 dollars. It's just not worth it to me to get crazy over something like this and what I did do worked out just fine. Would those of you who maintain that ALL attempts at theft should be met with beatings and vigilante justice be willing to stand up for that principal in this situation?