Form scripts may or may not have security measures to prevent being used by spammers to send out emails to third parties. ABVFP has quite a few. The most important is that it checks if the submission is actually made from your form, or by a bot script that then uses YOUR script to send the mails. It will also put limits to the number of @ , http and www in the fields. No spammer will lose time to use your script if he has to do it manually. So, just install it and you are done.
If you wish i can also implement what you are asking for: a captcha (that's how it is called): a dynamically created image, with numbers and letters, which the visitor has to copy in a field, otherwise the form can't be submitted. If you wish me to do so, please
contact me here. On my site, you can see some examples of our work.