Does your account still work and is it an administrators account? If so log in as yourself go to System Preferences->Users & Groups select the account your daughter made and delete it.
If you're locked out of the machine and have the install DVD's see Mac OS X: Changing or resetting an account password
Thanks Frank, I've removed the account but I'm a little concerned that she's changed her original account passwords so have put them back to the originals BUT can she access anything she was doing when in the new account via her original account? She's at school as I type but I will be telling her what I've done (and she WILL be grounded :-) )
If she doesn't have admin privileges she shouldn;t be able to access anything from the account she created.
Kids and computers, I know what it is like. Unfortunately if a person has physical access to the machine and knows a bit there is little you can do to prevent them from doing what they will.
One other possibility is to turn off your modem/router at night when no one is suppose to be accessing the net.
Good luck, look at the bright side if she is 15 in 3 years or so she should start acting like a human being again