Bolts and Screws
If you there are regular screws that bolt your license plate to your vehicle, you might want to look into some tamper resistant screws or custom security screws. Tamper resistant and custom security screws are a great option to prevent the common thief from stealing items off your car like your license plate.
The downside to standard "tamper resistant" screw bits is that anyone can buy the bits from a local hardware store. The safest option is to use custom security screws that use a custom designed bit that fits the custom made screws. The one downside is that the mininum order for custom security screws is in the hundreds.
BryceTM Fastener - This company manufactures custom bolts, custom nuts and custom designed screws for security applications.
Plate KeeperTM - Screws that require a tamper resistant screw bit to remove.
Tamper Proof Screw Company - Sells a variety of different standard tamper resistant screws and bolts.
Tamper Resistant License Plate Screws - Screws that require a tamper resistant screw bit to remove.