Register referrer to site

I need a script that registers referrers to my site. I want it to work this way: Each registered user can refer users to my site by linking to any of my pages and adding e.g. referrerID=1234 after the link: [login to view URL];referrerID=1234 The referrerID is the users ID in my phpBB forum. I would like to make sure that refreshing the url does not register multiple hits in the database. To prevent spam, that is. This must be done because each month the user with most referrals wins a prize. Registered details in the database should be the referrerID, referring URL, timestamp and other things that can be useful ;) Budget is $50. Payment is only via GAF, money is escrowed when project is started. In bidding, please write how will prevent spam upon reloading referrer URL.