Getting rid of carpenter bees
Spraying your hollies will not get rid of the carpenter bees. The individual nesting sites need to be treated.
But, the most effective deterrent to carpenter bees is a well painted, finished surface. By maintaining sound, finished wood surfaces, bees can be prevented from establishing nests around your home. Please see the following publication:
The cedar siding is stained with three coats of stain each one contains an anti bee chemical.The bees still drill their holes right through!
Yes, sometimes they can be difficult to deter especially if they like a location. The holes will have to be treated with a wasp/hornet spray as described in the previously attached publication.