CSS code would look like this:
#email {
	display: none;
}

OR if you're doing it by the div:

.specialfield {
	display: none;
}

Your form would look something like this:
<form method="post" action="yourformprocessor.php">
<div class="specialfield"><input id="email" type="text"></div>
<input id="realemail" type="text">
</form>