Sometimes my rat Frisco will become annoyed at me, if I'm bugging him by trying to get him to play with me, or sometimes he'll even get confused when he is trying to attack my other rat and get me instead.

I say "No, bad rat!" in a loud and firm scary voice, and then I flip him on his back and hold him there.

I don't know about pushing down on his muzzle. I would think that it might cause damage to his teeth or jaw.