Uninstall Directions for Linux/All Other Operating Systems

As root:
1) Run the commands:
su – zimbra
zmcontrol stop
exit
(you should be root after you run exit)

2a) Run the command: ps -ef | grep -i zimbra
If you see running processes
2b) Kill any lingering processes: kill -9 <pid>

3a) Run the command: df
If you see "amavisd"
3b) run command: umount /opt/zimbra/amavisd<-new-blah>/tmp

4) cd /<tmp_tar_install_dir>/zcs/

5) Run the command: ./install.sh -u

6) Run the following commands to complete the unistall:
rm -rf /opt/zimbra
rm -rf /var/log/*zimbra*
rm -rf /tmp/*zimbra*
rm -rf /tmp/hsperfdata*
rm -rf /tmp/install.*
rm -rf /tmp/*swatch*
rm -rf /tmp/log*

Ensure that you removed ALL the files owned by the user zimbra AND that contain the name "zimbra" in:
/var/log/
/tmp/

7) Run the following commands to delete the users and groups:
userdel zimbra
userdel postfix
groupdel zimbra
groupdel postfix

8) Remove the line "/dev/shm /opt/zimbra/amavisd-new-2.4.1/tmp tmpfs defaults,users,size=150m,mode=777 0 0" from /etc/fstab.

9) Remove the Zimbra logging lines below from /etc/syslog.conf:
local0.* -/var/log/zimbra.log
auth.* -/var/log/zimbra.log
mail.* -/var/log/zimbra.log

10) Remove logrotate config file for Zimbra (CentOS, RHEL)
rm -f /etc/logrotate.d/zimbra

11) Remove the Zimbra lines below from /etc/prelink.conf (CentOS, RHEL)
# added for Zimbra
-l /opt/zimbra/lib
-l /opt/zimbra/sleepycat/lib
-l /opt/zimbra/openldap/lib
-l /opt/zimbra/cyrus-sasl/lib
-l /opt/zimbra/mysql/lib

12) Remove Zimbra entries in /etc/rc* (CentOS, RHEL)
chkconfig --del zimbra

Other methods:(assuming you didn't already try the above) 
(dijichi2) if you get stuck and really want to just purge it from the system, try:
rpm -e `rpm -qa |grep zimbra`
rm -rf /opt/zimbra
delete zimbra entries from /etc/sudoers
delete zimbra entries from root and zimbra crontabs

On Ubuntu servers dpkg may think that Zimbra is still installed. Check by running dpkg --list (-l) or dpkg -q zimbra* and see if zimbra items are still listed. You can remove them by running:
dpkg --remove zimbra-apache zimbra-ldap zimbra-mta zimbra-spell zimbra-core zimbra-logger zimbra-snmp zimbra-store or

sudo rm -r /opt/zimbra