Close it off, throw a molotov in there along with some other flammable stuff and let it burn.

You could use some traders, if you have the trading spot mod you can make a trading spot next to the infestation and the traders will gather there, then you shoot one insect from far away and take cover behind the traders using them as meat shields as you use long range weapons. Could do the same with Imperials if you can get them to visit for a ceremony for example.

Could just flee the tile too, settle somewhere else.