We don't use a squirt bottle. We use a water pistol to remove ourselves from their sight and association with the negative reinforcement. When we DO use the water pistol, we try to be as unobtrusive about it as possible so that they have no idea where the water came from, but it seems to always happen when they're on the table or chewing on a cord so ergo
, getting on the table and chewing on cords causes it!
If I don't have a pistol handy, then the loud firm no and repetition. CONSTANT repetition. Narsil is stubborn, I have to correct him constantly; Mithril I can look sternly at and he'll stop whatever. Usually