A bit (say about 800mm to a metre) of one of your old chains attached to a comfortable handle is good - does wonders for windscreens if you ever manage to spot the mongrel, and it will roll up into a nice flat shape that should fit in one of your jacket pockets. If he stops, you can rearrange his face with it too.
I HATE people targeting us bikers - it's bad enough when they can't see you in traffic, to target you when they can see you is reprehensible and should be dealt with swiftly and assertively.

An alarm that has a wav screaming something like "Help, help, some bastard's trying to steal me" at about 120 dBm on a 5 minute loop should attract attention and embarrass the culprit...