LN would plausibly 1) create a long-lived hypoxic zone, and 2) have minimal effects on the groundwater, so I hand-waved that idea. Your boiling water would probably work through the "mud" they might use to line tunnels and their nest far quicker.
Let's start off with demolishing the houses with a controlled explosion that just implodes the structures so that we don't fire termites off into the blue. Carefully distribute the house remains to the perimeter. All crossings of the perimeter will involve a thorough decontamination.
Now we drill two metre (6'ish) deep holes every two metres with a diameter of at least 32mm (1.25"). Then we treat the problem:
Find the perimeter of infestation (may have to destroy more properties). Create a 3m (10') deep trench, 30cm (12") wide along the entire perimeter. Fill the trench with standard 1-2-3 conc. Make sure it is vibrated properly and use a viscosity admix. to ensure the trench is properly filled.
We'll need some heavy lift helis eg Chinooks - several are available from the Army and RAF or perhaps EH101 based jobbies - Army. We'll also need some containers and I think a 8 cu yard skip ie a standard builder's skip will do nicely. Yes we do have a wacky approach to units here in the UK. Fill the skips with boiling water and use the helis to drop the water on the area from above. Consider salting the water to lower the boiling point and make it more toxic in an environmentally responsible way.
Finally we go in with a fleet of ready mix lorries with pumps and long hoses. We pump a cementitious slurry into the holes and then finish with a capping slab of conc as required to support subsequent structures.
I'm actually half Devonian (mum born in Dartmouth) and I studied Civ Eng in Plymouth. I now live next door in Somerset.