It makes it hard when someone doesn't respect your boundaries and sometimes you have to be a little mean to them, even if you've addressed the issue and they pay no attention to what you said. Here's one way to approach this:
"Megan, I've told you to stop doing this (whatever "this" is) and you still do it. It makes me feel disrespected that you didn't listen to how it made me feel. I'm going to tell you one more time to stop. I don't find it funny. If you don't stop, you leave me no choice but to go to my parents and have them talk to your mom and dad. Which way do you want to handle this?"
Then follow through. If you threaten without action, it means nothing and she will keep bothering you. So say what you mean and mean what you say.
I'm guessing that if you approach her in private, look her right in the eyes and tell her to stop or parents will be involved, she will back off. Good luck.