Sadly there's nothing you could probably say to these girls that could make them change their minds.
My advice is to keep maintaining a positive image for sororities and greek life throughout all of your endeavors. Wearing your letters to class, while contributing to insightful discussions, can help others begin to put sorority girls in a positive light. I love it when people say "I would have never thought you were in a sorority!" And then proceed to ask me questions. Sometimes, girls even decide to either COB or try formal recruitment.
There is a large problem with prejudice of the greek system in general. Notions of "buying your friends" are prevalent, and others see sorority sisters and fraternity brothers and alcoholic partiers.
So that was a longer-winded answer to your question. One thing to consider is approaching the girls after class, explaining that their comments (especially the b*tches comment) were very hurtful to you. Explain that you understand that they did not mean to generalize, but that they may not have realized how offensive their comments really were.
And if that doesn't work, there's always slashing their tires
.