I don't see too deep into this problematic, but isn't the solution simple?
2-step authentication, normal thing nowadays. Wanna login to LFS account? Confirm via mail/SMS code/you name it. Wanna unlock the game? Same thing. I am aware the infrastructure required and the price of such systems might not be within reach of Scavier financial possibilities, but if it is, then isn't this the way? Steam has been working with this option for years. And that's only a drop in the sea, just to name one.