As a user you can always fake watching ads, there's no technical solution from the server side -- you have complete control of your browser (actually just controlling the display is sufficient). That's why websites don't even try to deny service for ad-block users -- it's a waste of time. So yes, I think a legal in the sense of law solution is the only way out. Unless you want to propose something like full DMR'd computers being the norm (essentially iOS everywhere), which I wouldn't want.