If you are just looking to stop this from happening in the horde/imp webmail included with sme...
/home/httpd/html/horde/imp/config/prefs.php
find and lock these items (locked should be true)
// user preferred email address for From: line
// see also $conf['hooks']['from'] in conf.php
$_prefs['from_addr'] = array(
'value' => '',
'locked' => false,
'shared' => true,
'type' => 'implicit'
);
// user preferred email address for Reply-To:, if different from From:
$_prefs['replyto_addr'] = array(
'value' => '',
'locked' => false,
'shared' => true,
'type' => 'implicit'
);
That will remove those options from the webmail system. Any entries already in the mysql preference database will have to be removed (it doesn't remove those items from existing users that have already set them)