making a form anti-spammer proof

How to block spammers from using email forms. I have a share with a friend form that I use on some of my sites so that a visitor can e-mail the page to a friend. It is written in PHP. I have recently had some trouble with spammers finding my scripts and sending spam through them. Is there a way to block spammers from using these "share with a friend" email forms for mischief?