Back when I had a roommate, there were a few times when we were going to have company over and we hadn't washed the dishes. Not wanting to leave the kitchen a mess, but being too lazy to actually wash said dishes, we threw them in the oven.
On at least one occasion, we left some tupperware thingamabob in there by accident when emptying out said dishes. Then we'd go to preheat the oven later, and there'd be a nasty smell and we'd know we didn't do a good enough job cleaning out the oven.
Just let the oven cool completely, scrape out all the melted plastic you can see, and use whatever type of oven cleaner works on your oven. The upside is that you'll have a very clean oven afterwards.