Carpenter Bee Dust
The best long term treatment for existing Carpenter Bees tunnels is to treat the entrance hole with an insecticide dust. The dust will remain in the tunnels for many months. In most cases this is long enough to kill any young adult bees that emerge from the nest. The easiest way to get the dust into the opening without making a mess is to use a hand duster. We prefer the B&G Bulb Dust-R 1150 because it has a curve tip that makes it easy to get the dust far into the tunnels.
See: Carpenter Bees for more information on using dust in carpenter bee galleries.