If you want to pay for customization, pomodoro sessions with blocking etc. I recommend Cold Turkey Blocker, but there is also free basic version. He block websites and apps, it's great!
PS: I'm not creator or a team member. I just love this app
Cold Turkey is great, and I believe it also makes it harder to unblock the domains once you have the blocks enabled. (If you're handy with tech and Google-fu you should still be able to get around it, though.)
I had no idea it comes with a Pomodoro timer. It's been a while since I've used it.