I've found a way to turn it off!
You need to download an application called 'multiclutch'. Install (it runs as a PrefPane) and then add keyboard short cuts for chosen applications:
By choosing a key combination that is not in use, multiclutch will get the gesture before the OS, and so will stop working for the gestures you want.
In this example setup for Safari and Preview, restart them, and job done!
And, this doesn't break other multi-touch gesture like rotation: