Thinking about this scenario; Definately challenge them, and loudly to draw attention to the situation. Don't touch the thief, but put your hand on your bike, this would act as a physical decleration of "this is my property". Tell the thief to stop and give him the oppotunity to leave. From there, if he doesn't stop (which I imagine would be unlikely.) It comes to reasonable force - you've given the guy the oppotunity to stop and walk away, he has ignored this and continued to act in a criminal manner. Try physically blocked him from getting at your bike. Step in front of him, or put your hands infront of his face. I would suggest you keep telling him to stop, and loudly to try and get as much attention as possible. At the point you block his work, he is probably going to start taking notice of you, so then it becomes the application of reasonable force to defend yourself. Presumably, this guy has some sort of tool that he is using to cut your bike lock, so he can be considered 'armed'. But as the last quote in my post above states, it's down to you to convince a jury that you felt sufficient threat to yourself to justify the damage you may have done to an attacker/thief.