How to clean molten plastic from a pizza stone?
For anyone interested: I ended up scraping the stone with a butter knife and some steel wool, removing all visible plastic, after which I put it in the oven and heated it up to about 275°C. After about an hour it stopped fuming, and from what I can tell there was not much left on it, although it was still kind of black, but not as black as it was when I put it in the oven.

There were no problems with the oven, as for the plastic, I am still not sure if there isn't some plastic in the pores of the stone, but since it is not fuming I guess that most of it is gone. I will use the bottom side of the stone in the future and hope not to poison myself with plastic.