Since black widow spiders spend most of the time in or on webbing, thorough applications of fine particles of insecticides, such as ULV, aerosols or fogs contact spiders in their webs causing quicker population reduction.
Although pyrethrin is registered specifically for black widow spiders, other space sprays of resmethrin (Vectrin) are useful for clean outs in outbuildings, crawl spaces, etc.
Many common insecticide aerosols will control the occasional spider. Some are labeled for homeowner use, while others are labeled only for the licensed, certified, pesticide applicator or pest control operator.
If large numbers of spiders migrate into the home, then a recommended residual pesticide may be used, such as chlorpyrifos (Dursban,diazinon or propoxur (Baygon).
Be sure to read the label and follow all instructions.
Spray the pesticide around the foundation outside the home and in the corners and along the baseboards of spider-infested areas.