That is very easy!
You have a /opt/zimbra/postfix/conf/read_from_ldap.pl script
This script it's executed every one hour (/etc/cron.hourly/...). The script read LDAP signatures information, and generate /opt/zimbra/postfix/conf/disclaimers/domain.name.html files.....
You can comment the line:
Code:
print TMPFILE2 "---------------------------------------------------<br>\n";