I finally discovered better method of rescuing an oven rack. The first step is to cool the melted plastic so that it becomes brittle. In colder climates, all this requires is stepping outside. Or you can try fitting your oven rack into a deep freezer. Once the plastic is cold, you can use a hammer to break apart the plastic.
To avoid warping the metal too much, you can use a wrench or another hard flat metal surface to brace the other side.
Then use the hammer claw to scrape off the bits of remaining plastic. Voila! No more plastic on your oven rack.
Have you ever accidentally melted plastic in your oven? How did you deal with it?