Once you replace the license plates on your car with new ones, the old plates are no longer valid. It's up to you to dispose of them so they can't be stolen and used illegally. The general advice is to make the plates unusable by bending them or destroying the stickers before taking the plates to your local recycling center. You can also turn them over to the state Department of Motor Vehicles (DMV), which will destroy or recycle the plates for you.
TL;DR (Too Long; Didn't Read)
If you're wondering how to dispose of old license plates, the best solution is to recycle them. You can organize this yourself or the Department of Motor Vehicles will do it for you.
Deface or Remove the Stickers
The first step is to erase all the information on the plates so they can't be stolen. If someone takes a plate and puts it on his car, the police will contact you if the thief commits a crime with that vehicle. The best way to deface your plates is with a permanent marker. Or, you can scratch the decals with a sharp object like a knife or a screwdriver. Peeling off the stickers is tricky, but if you can achieve this, it's a sure way to prevent the plates from being used in the future.
Cut or Bend the Plates Out of Shape
Modern license plates are made of lightweight aluminum so you can usually bend the metal with your hands. Alternatively, use tin snips to cut the license plates into pieces before you discard them. Older plates are made of iron and galvanized steel so you may need to go at these with a hammer or a pair of pliers. Once cut or bent, you can throw the plates in the trash or recycle them.
Scrap and Recycling Options
Your local recycling center should have an aluminum recycling bin, and cities that accept aluminum cans in the curbside bin should also take old aluminum license plates. Do check with the local DMV first, however, since some cities have special requirements for certain types of plates. In Washington state, for example, you can recycle disability plates and plates for veterans only through your local vehicle licensing office. Another option is to contact a local scrap metal dealer. You may be able to sell your old plates for cash on the spot!
Return Your Plates to the DMV
If this all sounds like too much trouble, the easy option is to surrender the old plates to your local Department of Motor Vehicles. The DMV will take your plates and destroy or recycle them for you. In most states, simply drop off the plates in person to your local DMV office or send them by mail to the address posted on the DMV website. In some areas, you'll have to destroy the stickers first and/or complete a plate surrender application, which you can download from your state's DMV website. Some county motor vehicle offices will charge a nominal fee to process the surrender.