Playdate help!!!!!

My DD is enrolled in preschool and has obviously made lots of friends through it.  I have no concerns with who she plays with at school, especially since there isn't much I can do about it.  However, one mom is always pushing me for a playdate with her and I really don't want one.  I know this sounds bad, but her DD is really poorly behaved and mom does nothing to control her.  She runs away, yells at her mom, basically tells mom what to do (and mom actually listens!)  My DD often mimics this behavior when around her.  I know they are young and 3 and some of the behavior comes with the territory, but she is just not the kind of girl I would like my DD to play with, and mom is not the kind of mom I would like to be friends with.  She is overly concerned with her appearance, financial status, and all things else superficial.  I know that I won't have control over who DD is friends with her whole life, but I feel right now I do have some say, and I would rather stray her away from bad influences. So I guess my question/issue is how can I NICELY continue to avoid playdates.  She attempts to make a playdate with me at least twice a week, and I have been really bad.  I avoid her phone calls or make up excuses.  I know... that is awful and childish behavior!!!!!!!  I just don't know what to do.  I don't want to hurt anyone's feelings, but I also cannot see us as friends.  Please help!!!!