You do not have to edit the install script.
Run the following to kill the cronjob:
rm -rf /etc/e-smith/templates-custom/etc/crontab/mrtg
/sbin/e-smith/expand-template /etc/crontab
Use the install script to find the names of each RPM that was installed, so that you can remove them properly.
For example:
To remove mrtg-2.9.17-3.i386.rpm
-use-
rpm -e mrtg-2.9.17-3