I've had success using a hammer. I might be worth hitting it a few blows with a hammer unless it was a very expensive on they tend to break from the inside where the u bend bit connects to the barrel. Might need two people so you dont break bits of bike as well.
If you wanna get scientific - we've got a bottle of nitrous oxide here which we use in the motorbikes - it'll freeze anything it touches in an instant (including your arm so be careful)
A meaty tap with a hammer will then shatter the lock