The password will have a life time. No one will want to put one in for every URL request. If they use the internet most of the time. It will always be enabled.
They could log off each time they leave it.
Hey a pig just flew over.
You could look at site banning software. It's a fair bit of administration work though, even with content recognition it will need tuning.
What's work related? Finding a hotel in city X is perfectly valid for a PA or HR, but probably is not for a data entry type.
Use the password to conrol what content you are allowed , even more work, and if the 'higher' level person's machine is open to access from other's they just plan their holiday on her access while she's at lunch.
They do this at my current place, and in my opinion it's a complete waste of time. A quick peek through logs / cache will identify problem machines, then a lookout and a monitor will catch them web handed as it were.
I know that one works, watched a fellow get escorted off the premises never to be seen again. He had a thing for ebony sites and the HR manager was of african descent and female.