So I did the deed last night, and I'm not sure if it worked. There were still some foragers coming in and out last evening, so I know I didn't get all of them. It was pretty simple, just poured the soapy water down the hole. I tried the hardware cloth idea, but I couldn't get it nailed down well because there is leaf litter all around the entrance hole, so I just scrapped that part of the plan, and it ended up not being necessary. The incoming foragers seemed confused by the soap at the entrance, but this morning they are going in and out just the same. Now obviously the hope for this strategy is to flood the brood nest, so it's possible that it's just a matter of time until the forager base decreases. I'm going to keep an eye on them for several days, maybe hit them again tonight with another gallon of soapy water, and we'll see what happens.