Excommunicate

This module will prevent all site emails from being sent to users who are blocked, or "excommunicated". The advantage of this module is that it doesn't require every single module sending email to check the status of the user before sending it.

Notes and Limitations

• By default, only the email address saved to the user account will be blocked. If your module provides other email addresses that messages could be sent to, you need to implement hook_excommunicate_mail_account().
• This module sets a custom mail backend (the "smtp_library" variable). If you are using another module that provides it's own mail backend, that module's drupal_mail_wrapper() implementation must call excommunicate_mail_wrapper() to send the message, or excommunicate_mail_send() to determine if the message should be sent or not.

This will start with a Drupal 6 version, but may end up with a D7 version if there's interest.