I... don't really think that some girl saying that you are "bad" should have such an important impact on your PC life. You should tell that girl to leave you alone, then ignore that girl, and talk to other people who, honestly, won't have any reason to believe whatever that girl is saying about you. The forums are full of people and I can guarantee you that she hasn't said that to more than 5 people, out of the 100,000+ members around.
Who is that girl, anyway? Because you might not be the first person she attacks.