Although it's not very cheap you can buy a supplement powder that you can give to your dog that works when the supplement gets into the dogs saliva and when they drool the saliva with the supplement helps to remove plaque.
The supplement is called ProDen Plaque Off and costs around $56.00 for a container of the powder.
It's costly but does have great reviews.
You might also be able to get the plaque off your dogs teeth by brushing the dogs teeth with some baking soda and water toothpaste.
You can also keep the plaque from coming back by regularly brushing your dogs teeth with the baking soda toothpaste.