How to block websites for a user, who has no root rights?

Question

The situation is: I don‘t want to able to surf certain websites out of distraction of boredom, while I am in the University, studying. My idea is:

1. Create a sudo-password which is to complex to memorize it, write in somewhere and leave it at home when I go to the University.
2. Create a user-account with restrictions, on the websites, which can be visited.
3. Study without distraction.

Answer 1

It's very unlikely that you can add any restrictions on other user accounts without being a super-user yourself. It's certainly not possible in Ubuntu's default and most other sane privilege management configurations.