Así:
Código PHP:
<form id="form1" name="form1" method="post" action="">
<input name="email" type="text" id="email" size="30" />
<input name="repEmail" type="text" id="repEmail" size="30" oncopy="return false" onpaste="return false" oncut="return false" />
</form>
(usando oncopy, onpaste, oncut)