People are like dogs. If you stare at one long enough, he’ll attack.
When you are giving a presentation, if an audience member gets aggressive and starts challenging you, resist the urge to spend too much time looking at that one person. Address his questions, but look at the other audience members, too, while you’re doing it.
When you keep the larger group involved, most individual attackers calm down. Their challenging questions might not end, but their hostility often will. They will relax, and you’ll keep a stimulating discussion from turning into a knife fight.