Easy fix:

JS doesn't let you change back button behaviour.

Q. But what about SPA?

A. Draw your own app-level back button top left of page.

Another solution: make it a permisson.

Yeah, no thanks. I want to use my browser’s standard keyboard shortcut to navigate back. And also forward again. And I want to be able to inspect the history listing before I go back or forward.

Let the browser do the browsery things. Don’t make SPAs suck even more than they already do.

Can I preventDefault on mouse5? What about the physical back button on Android?

>Draw your own app-level back button top left of page.

This is the worst idea I’ve heard all day.

Why not just put up a fake captcha page? When the user clicks the link to continue, the back button is now hijacked.