My US car has a standard 2" hitch receiver. The ball mount insert is stuck in the receiver. Its this type:
I can remove the pin easily. The mount wiggles around in the receiver (side to side) and can slide forward/backwards slightly (~1/8") with some force applied - such as tapping it with a block of wood.
But it will NOT slide out.
I simply can't generate enough direct force to tug it out; if I could get some leverage on it I suspect it could be forced out but there's not an obvious way to do that.
I'm guessing there is some grit in there which has trapped it like a wedge. It does have some light superficial rust but given the slight movement does not seem to be rusted in place.
I don't really mind if I had to destroy the inexpensive mount to get it out (grinder or whatever) but I don't want to damage the aftermarket receiver nor obviously the car itself. That said, I don't really see how I could cut it out without damaging other components.