I have a few carpenter bee holes in my siding. Most of these are holes that were treated last year and sealed up but are now open. I have an extermination company doing work for me. They are not the ones who sealed the holes though, and they tell me that they do not do that to the holes. They just spray a powder into the hole. I have had my house sprayed with and insecticide, although I don't know the name of it. I now have two different approaches to the problem and I don't know who to trust. What is the best way to treat for carpenter bees and to prevent them from coming back to my house again?

Either way is acceptable. Sprays do a better job of preventing bees from boring new holes. The dusts do just fine to kill bees in existing holes. Don't seal holes until the fall.
Eggs in the tunnels will hatch and bore out in the late summer. Only then should you seal the holes.