Its the same thread as the screws that hold the drum on, you know, the 3 that are probubly all missing! I find screwing one of them into the hole and out again with a decent screwdriver (gotta get a good grip on them) followed by a couple of taps with a rubber hammer is usally enough to loosen the drum off. That said my drums are removed fairly often so aren't as stubbern as some.
