The gestures occur at the OS level, so they can’t be interfered with by JavaScript.
More generally, the multitasking gestures are unreachable from anything in the app layer, so JavaScript would be unable to get out of the browser to disable them. (Even if you wrote a native app, you can’t work around them yet.)
I’d also argue that trying to interfere with the gestures, which the user has enabled and expects to work universally, is poor practice, and should be avoided.