I won't physically stop the person, but I will say something to them and take a photo of them in the act. I don't have a way of knowing if you're doing this legitimately (I've had to cut a lock due to a lost key), so I'm not going to threaten them or call the cops.

I don't think taking a photo will have any actionable follow-up—I have no way of following up to see if someone reports to see anyone has reported a stolen bike in the area—but might make a thief decide to go find a different target, while not being a major issue if you're the actual owner.

As a healthy, medium-large guy who's okay at de-escalation, I live in the assumption that no one will try to hurt me for interfering. I'll probably learn better one day.

Clearly he is determined to his objetive...