Maybe a way to get around this is to remove the links (not the text of the links, but the link functionality itself) unless you have an account and are logged in.
I guess in the end, if you don't have the self-control needed not to throw away your time reading about things (instead of doing things), you probably aren't going to benefit from being blocked from HN. You'll find something else equally unproductive to fill that hole.
Maybe an inline comment targeted at the user - in red or otherwise highlighted - saying: "you've been reading HN for X hours today, maybe it's time to enjoy some sunlight?" or similar text :)
HN is a good personal investment if you don't take the "fake work" bait[1]. Any non-fiction book is the same - you can spend hours reading every word/sentence/paragraph/page, or scan and focus with a purpose. It's easy to fall into passivity.
[1] Perhaps there is a better system to bring "good investment posts" to the top.
I predict shutting down HN for a few hours a day would make us even more addicted.
Might not be that bad if you work at night - Aus time.