Force against Force equals More Force (Ashanti proverb)
There is nothing more annoying, more frustrating and more gut wrenching than interacting with someone who has a racist mindset. The most interesting thing is that they don't even realise they are 'racist' - because this disposition exists in their own minds on a sub-conscious level. Such a person, if confronted about a racist slur, ignorant remark, insensitive joke, mindless comment or ignorant viewpoint, would deny to the high heavens that they are racist.
People don't realise that racism or racial discrimination exists not only in a physical form (actions and attacks) but is a reflection of fear and insecurity on the part of the racist individual. A fear of the unknown, a lack of a sense of belonging that manifests itself in psychological transference. Therefore one who makes a racist comment towards another is in actual fact showing that they feel intimidated and thus have to resort to less tangible or significant sources of dominion.
I have learnt to ignore such individuals. I would imagine one would be tempted to hit back with an equally stinging remark. Wrong move. After all, to dignify such remarks with a response only makes it more difficult to distinguish between the two (they do say it is wise not to argue with a fool for no one will be able to tell the difference!).
As hard as it may seem, the best response is none at all. In this silence, the point is bound to hit home.