Genes post with regards to the bees is spot on. They dont eat the wood, they merely use it as a place to lay their eggs. The female bores in from the side of a memeber then will hook a 90 degree turn and bore a centered channel or corridor up the length of the member. The corridor can be 2 to 4 feet long and will have branching "rooms" off that corridor. She bores the main corridor and lays eggs in chambers off this main corridor. When the larvae are hatching you can often times hear many many bees in a member. The female uses sound to bore a hole up the center of the timber and can pretty much eviscerate a 2x6 internally. Often if you hear the bees inside after hatching and you pound on a member, you will watch dozens of bees fly out of that single hole.
Unfortunately, injecting powdered or liquid poison into the hole and plugging it is still merely a symptomatic resolution. If your going to take the time to plug the hole dont waste your money on the poison although your likely expediting a slow a miserable death, your just throwing your money away. The hatched bees are not going to chew their way out.
Even killing off all the bees on your property is only going to help you in the short term. Maybe not even an entire season as new bees will come in to take over the area the instant the others are dead. This is why you see the male bees defending their territory so aggressively. They fly outside the holes that their females are working in and defend the female endlessly. There are endless other bees willing to take over as soon as those that are killed off are no longer defending the area.
Coating or treating your wood is not to say your coating it with a pesticide. Your coating it with a material that may hopefully make the bees look for other sites to lay their eggs. Paint, freshly applied penetrating solvent finishes, and so on, have been successful in making the females look elsewhere to bore their nests.
Its unfortunately one of the situations where our building practices of old will have to be changed because there are, and will continue to be, more bee's than the average homeowner has time to powder and plug holes to control. The solution is to build and use materials that put the bees in the woods as they would normally be. Its no different than a wood pecker boring a hole in your fascia. Its a pretty good indicator you have bugs in your fascia the woodpecker wants to eat. Doing away with the woodpecker still leaves you with the bugs and a material that bugs like to inhabit. Shortly there will be another woodpecker right there trying to get the same bugs.