It appears that you unassociate the TiVo from within the Amazon account. I just did it (and had a bit of trouble re-associating it, mind you).
How did you connect your TiVo to your Amazon account if you don't know the password for that account? BTW, the username for an Amazon account is your email address.
How did you connect your TiVo to your Amazon account if you don't know the password for that account? BTW, the username for an Amazon account is your email address.