I had the same problem on my deck which was built a couple of inches over a concrete pad. I tried a propane fogger with insecticides. It did nothing. Since the boards are spaced one inch apart you can't get anything under there to spray the nests directly even if you knew where the nests were. There is no one entrance or exit to trap them as there are hundreds of feet of spaces between the boards. Just spraying insecticide down the cracks won't hit the nests because they are on the bottom of the boards.
I finally solved the problem by tearing down my deck.