Hi IJsbrand,
Good question. From your iPhone, the best way to cancel is to either go to the main Lawncheck site, or the phone site:
http://cell.lawncheck.com Login, select your controller and then from the actions tab within LawnCheck, click the "Stop a Cycle" link.
From the phone site, click "Reset" and then click "Begin"
The next time your controller checks in the reset command will stop the cycle.
This provides Internet control. We are working on an iPhone app that will provide instant WiFi control but this is not yet available.
Hope this answers.
Jim