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…
This depends on your situation, read & click further on the deletion link.URLS: http://apps.google.comhttp://www.google.com/support/a/bin/answer.py?answer=1257646