One easy way to do this is to completely remove the sliding latch from the lock, as shown in the photo. Use a piece of strapping tape to hold the door closed.

Another way of disabling the lock is to mount the lock mechanism onto the door tabs in the reverse of the correct orientation. This will prevent the teeth of the latch from engaging the metal rim of the coin bank. An advantage of this technique is that the sliding latch piece can't get lost because it stays with the rest of the lock.