I had hornets nest under a yard storage shed a few years ago with the entrance under the doors so I couldn't get in without chancing getting stung, I thought on it for a couple days on how to get rid of them, what I did was took a length of 3/4 copper tube that I had and put a piece of masking tape over one end and then filled the tube with malathion powder, I stuck the tube under the shed where I thought the entrance was and then using a air gun I gave it a blast, blowing the tape off and the powder out of the tube,  the white powder boiled out from under the shed so I knew I got most of it underneath, a couple days later there wasn't a single hornet left and have never seen another one near the shed.