Perhaps you should consider some basic obfuscations: use non-standard names for form fields (for example, do not just use the "subject", "send", etc.). The less your form resembles the standard email submission form, the less likely the standard scripts will work on it. Of course, this will not stop a certain spammer. You can also prevent search engines from indexing your email sending page to make it harder to find spammers in the first place.
I would also say that captcha (or recaptcha) is very important.