It's actually really simple. I'll attempt from memory, so typos are expected. You can always google for a more solid answer as I'm sure there're few tutorials out there.
html:
<form method = "POST" action = "post.php">
<input type = "email" name = "email>
<input type = "email" name = "emailb">
<input type = "submit">
</form>
Javascript (jquery):
$(document).ready(function(){
$(".another[attribute='emailb']").hide();
});
post.php:
<?php
if (!empty($_POST['emailb']))
throw new Exception("SPAAAAM");
You need to do it with javascript as most automated bots won't be using it. However, you also want to check for the number of forms sent in X minutes from that IP and few other options to prevent even more. Note: don't automatically dissable it, show a recaptcha as a last-line measure.