I wouldn't worry about them. A few thousand people get bit by them every year but there hasn't been a fatality in decades. They're very unlikely to bite and very unlikely to cause any harm at all even if they do bite.

Maybe get some sticky fly paper though.

What eats them? (Really, from a distance they seem unbeatable.)

Daddy long-legs are known predators of Australian red-backs, while the American cellar spider supposedly hunts black widows.

Wasps