Prevent webpage access using forms authentication

Dear experts,

I have used the following two articles to setup forms security with roles.

http://support.microsoft.com/kb/301240
http://support.microsoft.com/kb/311495

Now I can use the roles to access or not give access, but I am wondering how to do this best.

I can show or hide menu items bases on the role, but a user can then still access pages if he/she manual enters the url manually. What is the best approach to prevent users from accessing a webpage?

Best regards,
MB