How do I block individual email accounts from sending mail to the server ?
This can be done by adding the account to the badmailfrom file located in the /usr/local/psa/qmail/control/ directory for the standard build and /var/qmail/control/ directory for the RPM build.
~# cat /usr/local/psa/qmail/control/badmailfrom
qmail-smtpd will reject every recipient address for a message if the envelope sender address is listed in badmailfrom. A line in badmailfrom may be of the form @domain.tld, meaning every address at host.
Alternatively you may configure Server-wide Black List through Parallels Plesk Panel.
Server -> Mail -> Black List
Main Menu -> Settings -> Spam Filter Settings -> Black List
NOTE: Parallels Plesk Panel 8.x and Parallels Plesk Panel 9.x with QMail MTA allows to add only domain names to Server-wide Black List, email accounts cannot be added.
In Parallels Plesk Panel 9.x with Postfix MTA it is possible to add email addresses to Server-wide Black List.