I use a tweak called KillBackground. It puts a nice red kill-all button at the bottom left or right (your choice) of the app switcher. One touch and all open applications are closed.
Download Remove Background and setup a gesture for it using Activator. My setup: Right-swipe on the status bar kills all applications except Phone and Mail.
Cheers guys but in the deck settings panel under custom actions there is a kill applications option. Which I would like to use but it doesn't seem to work