Real world solution is to let them know that that's your bike. The vast majority of bike thieves have no desire for confrontation and will run away. Police don't give a damn about bike theft, while they will put investigative effort into assault. Most thieves are minor criminals who will only get arrested if they're unlucky enough to get caught in the act, and have no desire to actually be the subject of an investigation.

While having your pepper spray ready is a reasonable precaution, it's not likely to turn physical if you don't start it.