We did a couple things when my son started this. First time we gave him a spray bottle with nothing in it. We told him it had invisible monster spray, only monsters were afraid of it and he could spray it at them and they had to leave...I've seen variations where people have put a little glitter in it with water, but our bedrooms are wood floors. Another time he started this he would come running out of his room and say there was a monster in his room, I finally got tired of telling him to use the bottle. So I asked him to talk to the monster and find out what he wanted. This went on for about 15 minutes, he would come out and tell me the answer to what ever I asked, and go back with the next....
So needless to say his monster named Michael just wanted to watch TV with him. After that occassionally he would come out and say something about the monster and we would remind him to ask what the monster wanted and who it was, and if finally felt comfortable enough to tell the monsters to just go away he didn't want to play or didn't want company that night.
It worked because he felt in control of the situation and I also told him that the monsters couldn't stay if HE said they had to go away. He was 4 it worked.
Good luck
Chelle E.