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.

For example:
~# cat /usr/local/psa/qmail/control/badmailfrom
@domain.tld
~#

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.

version 8.x:

Server -> Mail -> Black List

version 9.x:

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.