Channel your inner Temple Grandin and come up with some stock responses:
We aren't talking about that (something in the past), we're talking about (what's going on now).
And you bring that up because?
Would you like to rephrase that because I don't...
Thank you for sharing.
Talk to the hand.
Whatever.
That's an interesting point of view, but I don't agree with it.
That seems harsh.
Go fuck yourself.
Excuse me, but that is offensive.
You get the idea. Practice them in your head until they become second nature to you.