IMHO, it all boils down to communication. Perhaps your daughter hasn't understood really well your reasons why you are against of her having a Myspace account. Teens nowadays have the tendency to do exactly the opposite of what their parents are telling them. Maybe we can call these actions as rebellious, but that's just how they act right now. It is also necessary that you get her side. What made her do the same things again? Perhaps it's highly important that she can also air out her side and that on your part you also listen and try to take in her reasons. In the end, maybe a compromise would work: that is, you allow her to have her own MySpace account as long as she can prove that it wouldn't do any effect in her school work. Anyway, perhaps this can help you:
Advice for Parents.