Carpenter bees - ZAPPED!

[pomtzu]

Has anyone ever had a problem with them - and how do you get rid of them permanently?

A couple of years ago, we put an addition on the house and did some renovations - built a new front porch and a small porch at the entrance door in the addition. It's pressure treated wood, but has never been painted or stained, since we wanted it to weather naturally. No problem till this year.

Yesterday I noticed a small pile of sawdust on the front porch and couldn't figure where it came from, then I noticed what I thought was a bumble bee hanging around and I watched to see what it was doing. Sure enough, it went up under the edge of the railing and into a nice little round hole that it was boring out. Then this morning I noticed sawdust on the back porch and found a nice little round hole under the railing there too. I Googled and found that they are carpenter bees, and they bore and nest in wood. It said they don't like painted or stained or pressure treated wood, but these guys do. So I had some wasp and hornet spray and took that and saturated the bored holes and all around the area, and I'm going to spray again tonight.
The other back porch at the mud room entrance was stained after it was built several years ago, and there's no evidence of them bothering that one.

I read of one suggestion where it said to saturate a piece of cork with insecticide and plug the hole, but what's to keep them from boring out more in a different spot? Help! Any suggestions or proven remedies?