We had the same problem with my wife's Encore. Dealer didn't know how to do it. This will sound weird but here's the procedure:
Turn the key on but don't start it. Make sure there are no USB devices connected. Turn the car off. Open the driver's door fully and leave it open for 5 full minutes. If you have a proximity key with a Start button in the car take the car in the house away from the car. This will force a reset of the blue tooth. Start the car after the 5 minutes and try to delete the old devices again.
