How to remove your Google Music account
From the Android Market FAQ on http://www.google.com/support/androidmarket/bin/answer.py?hl=en&answer=1186964&topic=1100184:
To delete your entire library and withdraw from Google Music, visit http://music.google.com and select ‘Settings’ > ‘Withdraw from Google Music’.
Direct removal link: http://music.google.com
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Google Music using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Google Music. Click here to find out how.