If you just want it removed from your garage, just destroy it. Drive it into the ocean, blow it up, whatever. It will then be permanently removed from your garage, as long as you don't make a claim for it with the insurance company.
I accidentally got a second Elegy that I didn't want, so I parked it next to what I thought was just a random Infernus someone had swiped and left in the middle of the road across from my apartment (as I said, it was sitting in the middle of the road, really beat up, and no players anywhere in that part of the map), and tossed a grenade between them. Turned out the Infernus was a personal vehicle, but at least the Elegy is gone.
There are 10 types of people in the world. Those that know binary, and those that don't. GT:Talon Omega. If you send an FR please tell me where you got my GT.