Never.
If they want to go back let them.
If you disable the back button, your game will be unpublished. The back button is a sacred right of readers and anyone who disables it for even one page will have their game unpublished. No tolerance.
They can also just press delete in some browsers, so it doesn't work very well. This has been tried before, if you want to publish the story, publish it on another site.
It's not really possible to disable it.
ALT + Left Key Arrow Pad.
[Backspace] or abuse the save system. You can't really block it and if you did people would be pissed at you and hate your story for it.