How to block a Website using C#

Question

Hai
 I am developing an C# application(not asp.net) in which I have to get the urls typed by the user in the browser and I have to compare the urls with a set of urls and decide whether to allow or to deny the user to navigate to the website.
I have tried with appending to C:\windows\system32\drivers\etc\hosts file but its not working.So Is there any way I can do this?How to block the sites by getting the urls.Can someone suggest me how to approach to the solution with some sites or code that can help me.

Thank You
Chaithu