> If you can't prove it didn't happen, denying the accusation is libel. Fair is fair.

Yes, Bill Cosby was sued this way. He was accused publicly, he denied the accusations & they sued him for defamation. Everything depends though, it can be harder to show a denial caused damages or is not an opinion ie; I disagree with the characterization of events.

> Maybe the real problem is firing someone from their job due to non credible accusations

Shouldn't an accusation have real potential consequences, like employment termination? In that case sue the accuser for libel.