The two most obvious options for a latching mechanism would be a solenoid like this one: http://www.adafruit.com/products/412
or a servo like this one: http://www.adafruit.com/products/169
The solenoid would work like a deadbolt. Arrange things so the solenoid's slug runs through holes in both the locker door and the frame around the door, then apply a signal to pull the slug back and allow the door to open.
The servo would act more like a lock cylinder. Rotate the horn to fit in a slot that locks the door in place one way, rotate the horn out of the slot to allow the door to swing open and shut the other way.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.