First off, I wouldn't assume that this is always a prank. This is a rather infamous tactic used by bike thieves. These thieves add a second lock to "discourage" the owner from taking their bicycle, wait a few days, and then remove both locks, thus stealing your bike. Don't wait, get your bike out right away.
As long as this is your own bike, you don't have much to worry about. It would generally be helpful to call the police, so that they might make a note of it. They might not be able to help you cut the lock, but they will make a note of the incident. It might also be a good idea to register your bike.
Getting into the law part...
If you were to ever be charged with a property-related offence (which I doubt would happen), you probably wouldn't be able to be found guilty. In Canada, the relevant section would be §35 of the Criminal Code.
To summarize that, it basically means that you can't be guilty of an offence if you believe that another person is about to render your bike inoperative (through addition of the second lock), and that your act that constituted the offence would be preventing or stopping that. Don't forget, the bike has to be yours as well.