Tell them it's a joke and to get over it. I usually try to relate it to a dead baby joke.
Dead baby jokes are funny because they are offensive, my favorite is:
-What's the difference between a dead baby and a sandwich?
-Give up?....I don't have s3x with a SANDWICH before I eat it!!
Now obviously I don't have s3x with dead babies, just like I obviously don't think if I throw a basket ball it will stop five black guys from raping a white girl.... It's just stupid and offensive...but funny.
So that's what i tell them, and it usually leaves them with the "WHAT" look on their face....and then I change the subject.