I set a simple cron job to do just this. Weekdays from 9-5, Facebook, Reddit, and a few other sites are routed to localhost. Getting around it is more of a pain than just opening an Incognito tab, which is all that's required to get around a blocking browser extension.

Just having that schedule imposed on me for a few days was all it took for me to get out of the habit of opening a new tab and heading to one of the aforementioned sites any time I ran into a difficult problem. I've noticed an increase in my productivity, and I don't really even feel the need to twiddle away that time anymore.