A laudable goal that’s very important. I’m not aware of any service but, like a number of people suggested, if you make it inconvenient enough to visit these sites it could very well curb your addiction each time you’re tempted. I’ve found anything over a couple of minutes to by pass is enough to sour me on the idea of it

Someone suggested router changes to block them, I’d like to add to that, if your router supports it, setting specific hours of the day these sites are able to be used might be enough.

You could still password protect the router interface and make that inconvenient to get at in some way also.

We don’t have any Radio Shacks anymore 😞