Run following command on shell to start/stop mailnam service on cPanel server.

/usr/local/cpanel/3rdparty/mailman/bin/mailmanctl start

/usr/local/cpanel/3rdparty/mailman/bin/mailmanctl stop

Also you can login to WHM > tweak settings and Enable/disable mailman there.

How to check if mailman service running or not ?

ps -ax|grep mailman