This was an interesting question that came up here at least 4 years ago. (In fact it might even have been on the old MSNBC board.) There has been a lot of turnover in posters here since then so I thought I'd bring it up.

It seems to me most kicks and punches are out of the question. My choice is Brajilian Jiu Jitsu. It is a grappling martial art that also allows striking. A BJJ guy could hold his opponent with his legs and use that anchor to strike without flying away. There are also numerous finishes such as arm bars, knee bars, ankle locks, chokes, etc which could be performed in zero gravity because they are leverage based and not gravity dependent.

Then again, Sumo guys might be dangerous with all that mass.

(Btw, this isn't a completely pointless post. As more people (thrillseekers) start going into space in the coming years and decades, inevitably there will physical confrontations. Hell, given enough time there will be zero g prize fighting!)