We used to use an actual propane torch to wipe out black widow nests in California. You didn't need to get close enough to set the house on fire, just close enough to fry the webs and anything in them. A pair of welder's gloves added a measure of safety from scorched spiders if you got a little too close. http://www.bernzomatic.com/index.html
Those things would build their nests anyplace they could get into. They weren't a big problem in the Bay Area, but in the Central Valley, they were everywhere. Every time somebody had to go under the house, they'd make the first pass with a propane torch.