To allow or block URL navigation based category you need to have "site advisor content filtering" extension installed on ePO.
One extension is installed, navigate to "content action" policy. On content action policy page you can select category "Pornography" and set action for green , yellow and red rating for that category.
Please take a look at below link
To answer 2nd part
> SAE doesn't block URL based on specific content present on the page
> SAE doesn't modify page content either