Three checks. 1) no value for hidden html input, 2) updated value using javascript, 3) 5 second minimum required to fill out a form.

spam_prevention.html

<form>
	<!-- If this has any value, ever, it's spam. Human's don't fill out hidden inputs. -->
	<input type="hidden" class="spmChkOne">
	<!-- If this has anything other than 'ntSpm', it's spam. Bot tried to fill it out sans js. -->
	<input type="text" class="spmChkTwo" value="spm">
	<!-- If this has anything other than 'ntSpm', it's spam. Humans need more than 5 seconds. -->
	<input type="text" class="spmChkThree">
</form>


<script>
	$(function() {
		/* 
		=============================
		Prevent sp@m
		============================= */
		$('.spmChkTwo, .spmChkThree').hide();
		$('.spmChkTwo').val('ntSpm');
		spm5sec = setTimeout(spm5secGo, 5000);
		function spm5secGo() {
			$('.spmChkThree').val('ntSpm');
			clearTimeout(spm5sec);
		}
	});
</script>