That or fill it completely once with water, let it sit a while, them dump and repeat until nothing but water is coming out of it. When I say completely, I mean till it backs up into the shower through the drain. This will make sure any nests in it is submerged and will break down in the water. Then check and clean the vent.
Might not be a bad idea to run some water through the sinks and shower drain to make sure the p traps are full of water first so you don't get any wasps flying up through the drains to escape the flooding.