Scrape it off with a tough plastic knife or tool. The plastic will stick to the inside a little bit, but it usually lifts right off with a little elbowgrease. It helps if you turn on the oven for a few minutes prior to start, not long enough to get hot, but long enough to get slightly warm. This will soften the plastic so that it lets go easier. If you get it too hot, the plastic will melt again and you do not want that. Just warm enough so that it is not uncomfortable to reach inside for several minutes.
If you have a couple of small pieces left in there, they will burn off, but try to get as much as you can. I recommend after scraping that you set the oven to cook for an hour or so with nothing in it so that the next time you cook it does not taste like plastic.
You could also use steel wool on the oven shelves. Also, putting any oven shelves over a fire outside should burn off the plastic.
I haven't found an "Aggie and Kim" answer!!