How to disable the users' possibility to change their own email-passwords using the Webmail/Horde?
If you edit the configuration file
/usr/share/psa-horde/passwd/config/conf.php ( $HTTPD_VHOSTS_D
/webmail/horde/passwd/config/conf.php in Plesk version older than 7.5.3) and add some users to
$conf['user']['refused'] array. They will not be able to change their own passwords in horde IMP.