I had this one, big time, with ds. Had some excellent advice from MNers which was "ignore, ignore, ignore". At this age, any reaction, even a negative one, is good and means they will keep doing something. I used to just cut dead, turn my back, tidy the papers but just ignore him (but not leave him) whenever he did this. Whatever game we were playing just stopped until he was nice again. He didn't like it much but it worked! They don't understand "no" so that isn't really going to work, it just surprises them into stopping momentarily. Good luck