According to FAQ and SETUP pages, you don't need to cancel anything. The application is needed to run in the background. If you turn it off or uninstall it, then everything should return to normal.
There is a setup page where you can see how to setup your account. You can reverse the process to deactivate the option: http://www.phone-leash.com/setup.html#
On the bottom of the page, an email is also posted. Use it to get in touch with the developers.