Hiyaz!
Okay! Well, luckily, I've never had this problem(*knock on wood*)..but here are some suggestions:
~Make your account "No Chat". This isn't fun, but, this way if she does find out your password again, she cannot ban you.
~E-mail Fantage. They can handle it by banning her/doing something.
~Change your password. My password is SUPER long, but I remember it. Make sure it has capitals, symbols, and numbers to protect people from guessing it. HINT: Don't make your passoword: password or your username. My friend had her username and someone went on her and changed her eyes, skin, and clothes.
Hope this helps!