You've written about a cross that we liberals just have to bear. The conversations you report have been going on for decades. As several other commenters below have indicated, there's not much upside to confronting the idiots directly. It's not like our clever retort will have them saying, "Gosh, you're right. How could I have been so misguided all these years? Excuse me...I'm going to send a check to the NAACP right now."
I just use the "thousand yard stare", and the "no visible response" method, often with the blatant and overt "change the subject" gambit. It works like this....someone says, "It's the Mexicans and the Blacks." I look at their forehead or at the wall behind them with no acknowledgement of what they said...no smile, no head nod, nothing. After a moment, I say "Gosh, how about them Browns?" (my local NFL team). It works especially well if it's not football season.
Pretty quickly they get the message that I'm not going to engage in their bullshit and they quit bringing it up around me. Your mileage may vary but it seems to work for me.