On the importance of NO :

4. Teach your kids that “no” and “stop” are important words and should be honored. One way to explain this may be, “Sarah said ‘no’, and when we hear ‘no’ we always stop what we’re doing immediately. No matter what.”

Also teach your child that his or her “no’s” are to be honored. Explain that just like we always stop doing something when someone says “no”, that our friends need to always stop when we say “no”, too.  If a friend doesn’t stop when we say “no,” then we need to think about whether or not we feel good, and safe, playing with them. If not, it’s okay to choose other friends.

If you feel you must intervene, do so. Be kind, and explain to the other child how important “no” is. Your child will internalize how important it is both for himself and others.