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’.
URLS:
http://music.google.com
http://music.google.com
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