Letting people get away with immoral acts only encourages them to continue acting immorally - and their victims likely won't be limited to you.
The key word in the legal arena is deterrence. People are punished by the law at least partly to send out a message.
There's also a matter of self-respect and setting boundaries. In that spirit, I'd take the bike back...with interest.
However, from a practical perspective you have to bear in mind that you could be legally charged with trespassing, and possibly even theft. This highlights one of life's big ironies: Why is it that bad people so easily get away with their crimes?
I've worked for several employers where bullying and corruption was rampant, and the biggest, most obvious bullies were impossible to fire. Yet the brightest, hardest working employees can be fired at the drop of a hat.
I'm still trying to figure out exactly what makes this situation possible. But my personal advice is to remember that - in the U.S., at least - justice is never on your side. The game is rigged in favor of the rich and powerful...and apparently in favor of schoolyard bullies as well.