I need a configuration or plugin in my joomla site that will block visitors coming to my website that have come from certain sites. For example if someone has come to my site from a file sharing forum, then they should be blocked showing a message that I specify. My sources say that this is not a very difficult project at all and rather easy to implement. Some codes perhaps need to be put in the php. This implementation must be safe not not break the site or leave any vulnerabilities.