You can't take away the ability for the end user to not disable LogMeIn completely, but you can remove the LogMeIn Icon from the systray.
-From "My Computers" click on the "Configuration" link on the left hand side of the page.
-Select which computers in your account you want to apply this to, click "Continue"
-From the drop-down menu select "Appearance".
-Uncheck the box that says " Display the LogMeIn icon in the System Tray". Click "Save."
Once that system is rebooted, the LogMeIn icon will no longer be in the sys tray so then end user will not be able to right click and disable LogMeIn.