Plan B - Kill the Dog!
This is a hard read - and not specific to Yorkies. But as a large dog owner I am more than familiar with this problem.
I read here all the time about never correcting a Yorkie - well guess what - sometimes they need a measured correction. Measured to their size and transgression and done safely for the dog. It is quite simply wrong to allow a Yorkie to snap snarl and then eventually bite some-one.
On our last BRT walk I took aside two owners of Blackies and spoke with them pretty directly. Their breeders should have done this way earlier - actually before they placed this puppy with them. They needed to get themselves and their dogs into obedience training pronto! Two dog and people aggressive dogs - a heartache waiting to happen. And why? Because they did not know how to train this working breed. Did not recognize the seriousness of a problem waiting to happen. Only one to my knowledge took my advice. And then she spoke with me about not wanting to use the collar the trainer I referred her to recommended. I said talk to him - but really you should try it out - understand its uses.
Here is my ideal for a trainer. Someone who actually has enough knowledge and the belief system that you need to train the dog and owner as a team! Each owner dog team has their strengths n weaknesses - and it is a superb trainer that recognizes this and caters the training to this. Oh and btw put your ego at the training door - be prepared to hear about your mistakes or flaws and openly be willing to improve on them.
Sadly I can't make some one listen to me and to follow my advice. I am a novice trainer - and I say this having been involved with training dogs for 10yrs. It is only now I fly solo for the most part in training. But still I go to my professional trainer for review of our work and fine tunings. If I with 10yrs of experience still find the need to ask for help and advice - certainly new dog owners can do the same.