Permethrin. You should be able to get it at a Tractor Supply or Quality Farm & Fleet. Dilute it according to directions, or to about 10% overall concentration, put it in a sprayer, spray all around your garage. Follow all safety precautions, and wear googles and a respirator just to be safe. DO NOT allow any cats to come in contact with it until it fully dries; permethrin is very toxic to cats, but it should be fine once it's dry.

It will kill arthropods (spiders and insects), and acts as a fairly long-lasting repellent to prevent future infestations.

As someone else said, if you have a spider "problem", your real problem is that you have a lot of other insects that are acting as a food source for the spiders. Figure out what the other insect infestation is, fix taht, and you should fix your spider problem.

EDIT: black widow spiders aren't actually particularly dangerous to people. Most bites resolve themselves without any medical attention, and the very, very few fatalities are usually very young children, very old people, or people that are already very ill.