[gemy]

These techniques (some of them will need to be modified for the Yorkie's mouth size).

A sibilant "SSSSSSSSS" sound does work and tack on No at the end of it.

A two finger (maybe just one finger with a Yorkie); quick firm poke under the ear is very effective as well. It must be immediate and can be combined with the Sssssss sound. Try it on yourself first to get the pressure correct. It should be firm quick and immediate to be effective. It has the effect of "jarring" the dog out of the mindset it is currently in.

Most importantly do NOTHING that the dog can construe as a reward for his behaviour. Like pick them up, give them a toy, crouch down and talk to them. As a young puppy you can substitute your fingers for a toy; but at seven months old by now, they look at toys as a reward.

Those are just some ideas.