[quote name='Dingus' timestamp='1394814086' post='2395563']
It's not your job to change them or impress upon them them the error of their ways. All you want is their custom , not their friendship. Give them every opportunity to buy, regardless of their manner, and then politely show them the door.
[/quote]
I think I'd largely agree with this, but still: in my mind your job is not only to sell something but also to do it in a fashion that allows you to "keep up your self esteem" so to speak (I lack other ways of describing this better).
In some extreme cases, where obnoxious customers try to remove your dignity, I think it's a good idea to withdraw from the dealings, and probably to confront them - not in the way of "you must change your ways" but more like "I'm outta here".
To put some numbers to this: where I come from it's regarded as a given that 5% of customers are complete arses. I'd reduce that to half that number, and have personally allowed myself to confront 3 customers out of roughly 1,700.