How do you dispose of old plates that are no longer in use?

You would have to bring your plates to the lower level of the DMV by their in-house store. Directions: Start at their parking lot, enter through the Parking lot doors and continue ALL the way to the back. You will see two gentlemen sitting at a desk. That's where you would need to give it to and the will issue some paperwork

Thanks where would we bring them, to motor vehicles?

They are recycled