script to block ip address from filling forms out on website

I need some way to block certain IP addresses from filling forms out on my website, be it a program or code. I'm using a php script for the forms now, and the program allowed the ability to block certain ip addresses-but it does not seem to block them. What I would like to do is have an admin area where I can manually add addresses that spam, and anytime someone from that ip address tries to fill out the form they are taken to an error page. If you know of any solution other than this, I'd be more than happy to listen. thanks for bidding Mike Simmons