Although black widows are the most poisonous spiders in North America, their bite is rarely fatal. The bite does cause a lot of swelling and is quite painful, but does not have lasting effects.
The good news is that black widows are extremely shy and will do everything they can to avoid people, and, even when encountered, will only bite when provoked. As a result bites are extremely rare, especially when you consider how common (though rarely seen for the above reason) black widows really are.
Black widows prefer to make their webs in quiet, out-of-the way areas such as garages and attics. Their webs are not neat orbs, but a very random assortment of strands and often have a funnel in the middle. When approached, they scurry for cover.
If they really bother you, a pyrethrin or permethrin spray is the least toxic to Kerries and other vertebrates and most environmentally friendly.