I want to avoid my frenemy (former friend who is now an enemy) at all costs, but she is everywhere!?

I had this BF growing up, but once we reached adolescence, she started to change and she wasn't a good friend anymore. All she really cared about was being the center of attention, flirting with useless guys to get what she wanted, being promiscuous, putting her bestfriends off (i.e. myself) to be with those useless men, not going to school, doing drugs, and finally, starting to be just like her entire family (who happen to be really messed up). I realized this early on so ever since then, I haven't spoken to her since for fear that she would ruin my reputation (and again, her reputation has pretty much been ruined already). I've also tried my best to avoid her ridiculous mother, who has said some very unpleasant things about my own sister, even though her own daughter is so messed-up! So my questions is, what do you say to someone - that you don't like and don't want to be friends again - when you have no other choice but to encounter them again? I am 100% certain that I do not want to be her friend again, nor am I interested in starting a conversation with her because I don't want her to get any ideas that she can use me like she did before. I am very frustrated right now, because I need to prepare for that day when I have to encounter her again. Geez...this sounds so much like that MTV show "The Hills." Please give me an answer that is sensible to use in real life. 10 points to the best answerer!!