Every time to catch yourself doing it, shake out of it and continue with whatever you were doing. It’s going to be tedious and repetitive, but you’re basically reprogramming your brain to not do that, so it’ll take a while.
But what if I wasn't doing anything to start? My maladaptive daydreaming starts a fillers for the hours I don't do anything, and then turn into canceling plans and getting no housework or any productive things done. (Sorry, I'm trying to find help and answers and don't know where to start exactly)
@CyberWilliam You have to catch yourself daydreaming, obsessing, ruminating, etc. and stop doing it.