stop mailman daemons if system has them

crontab -u mailman -e
kill entire crontab
remove /var/spool/cron/crontabs/mailman

clean up mail aliases

move the list archives somewhere safe ($MAILMAN/archives)

clean up httpd config files

run through mail server install directions in reverse:
http://www.gnu.org/software/mailman/mailman-install/index.html

kill mailman daemon scripts, if any

kill mailman directory

userdel mailman
groupdel mailman