What to do if someone has locked their bike to yours?

Can I suggest that if you buy cutting gear to rescue people from these situations, you also invest in an extra lock... One so you can relock the offenders bike up with a note about how they can get it unlocked... A bit like clamping but doing it to a) stop their bike being stolen b) inconvenience them as much as they have done to others. Call it karma.

Obviously charging them a fee for release would be unlawful, which is a shame.

I thought this too might be appropriate because in the instance of the OP it was just an error... they happen. I wouldn't want to get someone's bike stolen for them because of their mistake.