you already have captcha on this form, so that will prevent spam getting through that is submitted through automated robots.
it cant prevent spam that is from a real person filling in the form though.
in the email body, you could add code to write the IP address of the person filling in the form:
<?php echo((isset($_SERVER["REMOTE_ADDR"]))?$_SERVER["REMOTE_ADDR"]:"") ?>
then when you get spam, you can ban the users ip address: