Boric Acid Products
One way to kill an ant colony is to have some kind of poisonous material, such as boric acid, combined with sugar type bait that the ants actually take themselves into the colony. Then the bait, when digested, kills the ants. There are a number of commercial roach killer products on the market containing boric acid (a safe natural material) as the primary ingredient. These products may also be effective against a colony of carpenter ants. There usually is some kind of sugar bait in the product that causes the ants to take the boric acid inside the colony. Because the workers feed many other ants in the colony, the boric acid is spread around and will eventually kill the colony. Generally, these products are only effective if they are kept dry, so presentation in an outside setting could be a problem.
If the ants are inhabiting a tree where the decayed section is below ten feet and appears to be reachable by the yardener without risking life and limb, then you can try removing any loose decayed material and drench the area with a safe insecticide such as Pyrethrum and Pyrethrin Insecticides or Neem Oil Soap. The problem with this technique is that if you don’t get the material directly into the main chamber of the colony, you will not kill the colony.
Carpenter ants found in a woodpile can be controlled by spraying the wood with insecticide since they are usually exposed when you start taking the woodpile apart.