As soon as you delete a server from our interface, following command will be shown to remove the monitoring agent and any related data as well as user accounts from your system. Simply execute it as root on the server you don't want to monitor anymore.
rm -R /etc/nodequery && (crontab -u nodequery -l | grep -v "/etc/nodequery/nq-agent.sh") | crontab -u nodequery - && userdel nodequery
To verify that everything was removed successfully, simply check if the directory /etc/nodequery and its owner account nodequery do not exist on your system anymore. If you are using an older version of the monitoring script, you might have to remove the cron entry with the user root instead of removing the account nodequery.