I have a new rule. I think it's a pretty good rule - feel free to adopt it if you'd like.
If I'm in a conversation and someone says anything along the lines of "I heard something about you. Or an opinion. Or an idea. Or a whatever." I immediately stop them and say "Will you be able to tell me who said this?"
If the answer is "no" or "well I can't say who said it" or "out of respect for..." then I will graciously decline to hear whatever was about to be said.
I'm done responding to imaginary people. I won't let that evidence be submitted into the court of my head. Too many real people I'd rather respond to.