I recommend just using a white list if it is that important to you. Blocked site includes a link to submit a site for review to be whitelisted. You could optionally review a week or month worth of web traffic and see what sites are hit the most often and add those to the whitelist ahead of time. Regardless it ends up being pretty much self policing. People won't request access to a site that they know is going to be reviewed first if they know it is not allowed at work. Review of submissions can be however frequent you think is reasonable, with the user having the onus to escalate if the need to view a new site is urgent.