Carpenter Ants: How Best to Kill the Nest?

Just found carpenter ants in the beams of an old building I am working in. How best to kill them?

All plaster on inside of exterior walls have been stripped of plaster and the cavities are all open. (That's how I found the ants: piles of sawdust in the cavities. Also saw a big black ant looking at me from nest hole.)

There was a leak in the roof for years near the nest, that leak was fixed a year ago. There may be other leaks around windows, these will be addressed soon.

Beams appear solid from outside and when hit with a hammer.

In spring there were maybe 2 dozen winged black ants (queens) buzzing against closed attic window. Is that a lot of queens? It may indicate the nest is dying due to leak in roof being fixed.

Total sawdust observed is about 1/2 cubic foot, maybe a bit more.

I will be spraying foam insulation into the cavities in a month. Will that help kill them? I assume poison should be applied before the foam.

This is a commercial building, no residential apartments in this building.

What is the right poison to use? Any other issues I should be aware of, such as method of application?

This is in the northeast.

I am calling exterminators but I know nothing about this and don't want to get ripped.

Thanks for your input and your votes.