Foam spray is the way to go - we also have many wasps, yet another reason I really like my resin structure on the pool, it is blow molded resin, so no crevices for the buggers to set in. Unfortunately, like anything, there's always the one that tries, they seem to only build inside the uprights, since the resin pool is held together with one big quarter turn lock nut on the top of each upright, it's as simple as taking the nut off and spraying down the hole filling the cavity with foam.