You can buy a Raid product that shoots a foam for quite a distance. Used it several times, aim it at the opening and fill the hole as the pests come out they drop dead almost immediately. Works good on hanging nests. The ones that got me were nesting in the seat foam.
Yeah, I used some of that, but it just didn't give me the same level of satisfaction as burning 'em! My can of Raid said to wait until morning as well (ala Randy's post above), but after getting stung I wanted satisfaction...NOW!
Spent the past weekend working on the soffits and gutters of our house, and sure enough there were some wasp & hornets nests up there as well. Used three cans of Raid, and left the "flamethrower" back in the shop this time.