I am asking a question on behalf of a friend...

He would like to know whether there is any solution (probably a software; think it is a firewall in my opinion) to block certain websites that he visits I think you can figure out that those websites that he visits are not good websites so he wants to be blocked from accessing them. The thing is that he doesn't want anyone to do it for him so that others will know the bad websites that he visits. On the other hand, if he does it himself by using a firewall and enter the list of websites to be blocked in it, he can easily unblock later.

This means that he wants a solution where he can enter the list of websites to be blocked from his computer and will find it very hard (if not impossible) to unblock them.
Does anyone know of such a solution? 

Thank you in advance for your answers.