I asked the dealer how to do this and the tech printed out a page titled Procedure to Disable Smart Key. I've tried it and it does work!

Start with the engine off, driver's side door closed and unlocked. The door to be opened and closed is the driver's door. You have up to 5 seconds between each step.

1. Unlock the car once with the unlock button on the remote
2. Open the door
3. Unlock the door twice with the remote
4. Close and open the door twice, leaving the door open
5. Unlock the car twice with the remote
6. Close and open the door once!
7. Close the door and the car will beep twice. Now the Smart Key has been disabled. Repeat this proceedure to enable the Smart Key, and the car will beep once when enabled.

If the car does not beep after finishing the proceedure, you took too long between a step. You can still open the car with the remote. But the button on the door handle, and grabbing the door handle to unlock the car will no longer work.