You can use a Dialog plugin in order to create a pop-up box to give the user an option to exit the game when the back button is pressed.
There's two dialog plugins that i've used, both work well.
- Cranberry's CordovaDialog plugin, which is what i currently use, but you have to pay to get access to the plugin.
- CocoonCanvasplus plugin, you can use the dialog feature but it only works if you compile using Cocoon.io & Canvas +, but its free to download.
There might be other dialog plugins, but these are the ones i've used so i can vouch for them.
Hope that helps.