Can't you just push the "down" button? Not sure, as the 300 is a little different, but on the 200 you could press the down button to stop it. Like when playing a P2 clip back, up=pause and down=stop, and the same metaphor applied in interval recording. But, the 300 does things a little differently so I'm not sure if that'll work; try it though.
Hi, if i remember accurate you can stop intervall recording by pressing the "Stop" Button at the Back of the Camera. Where the other buttons like play/Pause, forward etc. are.