A friend once advised me, to seriously and intently ask such people "what the fuck is wrong with you?" (and possibly shake your head a bit in a confused mildly disgusted manner). Then walk away, because whatever they have to say is not going to be worth your time.

The one time I applied this, it netted me a rather sincere apology half an hour later, so I guess it has some merit in at least some of the situations.

(note that I was not in the group being discriminated against, I don't think it'd be a good approach if I were)