Remove/Kill Wasps When You Cannot Get to the Nest
Have you thought about using a trap?
We had an infestation of yellow jackets last fall, and tried almost everything to wipe them out after my brother got stung by a swarm while out mowing the lawn, only we couldn't find the nests. What we ended up doing was hanging a trap in the back yard, and only a few days later, we started catching the little buggers in it.
The trap we bought, called a &quot;Rescue W.H.Y&quot; trap (make sure to grab attractant refills too), supposedly works on wasps and hornets, but I can definitely say that it works on yellow jackets. You can find them in the Garden section of a local Home Depot or Lowe's, but it's a little cheaper online, if you don't mind waiting a few days to get them.
Hope this helps!