You have to start teaching them on how to tell what you don't want them to do as well. If you don't want him to do that, just gently push head away and say "no."

Try not to keep switching your sentence like, "don't do that," or "stop it," because it will only confuse them further. They can't understand our language, after all. It's our job to teach them to understand.

"No," will do just fine. Keep saying it until it gets the idea that you don't like it. If he did it right, make sure to reward him. They'd learn faster that way and grow closer to you faster too.