Very simple and stupid chrome extension to block arbitrary sites

block.js

document.body.innerHTML = 'get back to work'

manifest.json

{
  "manifest_version": 2,
  "name": "dumb blocker",
  "description": "made by mortenson",
  "version": "0.0.1",
  "content_scripts": [
    {
      "matches": [
        "https://www.youtube.com/*"
      ],
      "js": ["block.js"]
    }
  ]
}