Stand up, turn around, look them straight in the eye -
"Unless you want this conversation to be the spark of a 6pm news story, I suggest you take this conversation elsewhere"
(Of course, this will only work if you aren't in an "overly corporate" atmosphere where sarcasm/pointed comments are not allowed)
To me, it is no different than if they were standing near you talking about "n-words". Some people will just never understand that their idiocy (be it racism, homophobia or animal cruelty) is just NOT a convo that they can have in public without consequences!