perennial termites

Didn't say where you were.  If you have subterranean termites I feel you will always have termites.  We've battled them for 4 decades.  It means inspections and treatment periodically.

We've had chemicals and experimented with something that they were supposed to take back to the underground home and then it kills all of them.  Nothing is forever, except with these creatures.  Leave a piece of nice wood on the ground for a few months and pick it up to find it half eaten.