A new thread on Microsoft's Xbox Live Support forum explains that an unspecified numbers of users have been banned from the service following an investigation geared around rooting out Marketplace content thieves. The post goes on to state that there is "clear evidence" against those who have been banned and are now stuck with "a permanent suspension from the service."
Obviously, the identities of individual offenders are not being revealed. Microsoft isn't even saying how many wrongdoers it discovered. It looks like the company won't be hearing any excuses though. If you've been hit with the banhammer, it's because someone at Microsoft has verifiable evidence that your account is linked with illegally obtained content. That's what the company's position is, anyway.