Here are many ways of keeping your house safe from the black widow and any other dangerous spiders such as the brown recluse.
Use the best pesticides in your garden to kill the black widow spider or brown recluse. You need professional pesticides --not the common ones commonly sold in stores, groceries or big home supplies-- in order to correctly immunize your house perimeter (use Cynoff WP, Demon WP).
The residual non-odor treatment of professional pesticides is the only thing that will certainly keep spiders away.
Use the same products to build a venom barrier against spiders in every place through which arachnids may enter your home such as doors and windows. Same treatment applies to the places where spiders usually hide such as under furniture, cracks, closets, window sills, etc.
Cobweb Eliminator is very useful in places where spiders can or do build their webs, such as around light sources, curtains, roof corners, etc. This product prevents spiders from rebuilding their webs.
Learn every inside aspect of the black widow spider following the links in the main menu.