How to disable/prevent emails being sent to a certain PHPBB user?

[Fli]

When i start receiving delivery failures when my PHPBB sends mail to a certain user which mailbox no longer exist, I would want to prevent any further mail to such user.

How to achieve that?

IDEAS (most if not all would not work and may not be tested):

A)
Go to user management in ACP (admin control panel),
A1) in the drop down menu select Settings form and set user not to receive messages/information from administrators.
A2) delete user (deactivating or banning via quick tools drop down menu may not work IMO)

B)
- Replace their email domain with some domain known to work as a black hole for any email account on it. If I can not found public one, I can setup own email black hole for my PHPBB forum domain in my webhosting control panel. Function is sometimes called Catch All and the action would be to discard email which is sent to a email address which has no account created. Negative impact of such configuration may be (according to a DirectAdmin) significantly increased server load, because email is said to be processed and then discarded. That way I can replace no longer existing PHPBB user email domain with my forum domain..

C)
C1) MySQL command to disable notifications (can be converted to a PHP and possibly placed to ACP folder for password protection?

UPDATE `phpbb_user_notifications` SET `notify`=0 WHERE `method`="notification.method.email" AND `user_id` = 123456

C2)

delete from phpbb_forums_watch where user_id = 123456
delete from phpbb_topics_watch where user_id = 123456

D)