The MediaRight entry is a cookie. If you clean/remove the cookie, it is deleted from your browser. Where you get the cookie depends on what sites you visit. I'm not sure if Spybot-SD's Immunization covers this (prevents it from being added to your browser).
Technically, you cannot remove it 'forever', as it reappears everytime during your browser session.
Another workaround would be to clear the cookies from your browsers after every session.