Hmm, have you tried closing the door with the cat inside? In other words, when the cat joins you in the bed, say, "oh good, Bedtime!" and close the door. If the cat cannot go out, usually, they want OUT NOWwwwwww........
Of course, they may be happy and want to stay, but..........
Speaking as someone who likes the cat in bed because they tend to sleep on my arthritic joints (which helps, btw), this works, so my door stays open..........