All very good and very relevant answers. however neither answered the question asked.
The first thing I'd do is you want to stop her using that account is to sign in with her password then go to account settings and change the password to something else that she does not know.
That way she cannot sign back into it.
After you have done that do this.
Go to
http://www.hotmail.com (sign in with your daughters username and password - by the way you can change the password here also in account settings)
click Help on any Hotmail page.
Type Close Account in the keyword search section of the Help screen.
Click the Close Your Hotmail Account tip within the Help screen.
Click the Close Account link that is highlighted in blue.
Click Close Account
If you want more facts on keeping your daughter safe online take a read of
http://www.fixmypcuk.com
Its got lots of uselful facts and tips, and even sells a cd that allows people to fix their own pc should you have a problem and its designed for people who know nothing about computers and is easy to use.