It depends on the app. I use SelfControl.app for Mac, which blocks a list of websites for a duration of time. It resolves the IP addresses of all blocked domains, then generates a set of ipfw rules and applies them to all interfaces.

You can disable the block, but it’s a huge PITA especially if you just want to disable one site. It’s a minimum five minute effort to unblock a site; for me that’s plenty sufficient motivation to get back to work.