<form id="form"><textarea name="com_text" onkeyup="aaa()"></textarea></form>

<script>
function aaa()
{
	var reg = new RegExp(/http:\/\/|a[^h]+href/g);
	
	if ( form.com_text.value.match(reg) )
	{
		form.com_text.value = form.com_text.value.replace(reg,"");
		alert('no spam!');
	}
}
</script>