Whilst it is against the agreed standards of use, you are asking a question many have asked before.
Put this in the head:
<script language="javascript" type="text/javascript">
function EnterTab(e) {
var intKey = window.Event ? e.which : e.KeyCode;
if(intKey == 13)
e.returnValue = false;
}
</script>
And in the body tag:
<body onkeydown="EnterTab(event);">