function fnIgnoreEnter(thisEvent)
{
    if(thisEvent.keyCode == 13) // enter key
    {
        return false; // do nothing
    }
}

<textarea rows="20" cols="50" onkeydown="return ignoreEnter(event);" onkeyup="noEnter(this);"></textarea>
<script type="text/javascript">
function ignoreEnter( event ) {
 return thisEvent.keyCode != 13; // enter key
}

function noEnter( e ) {
 e.value = e.value.replace(/n/g,'');
 e.value = e.value.replace(/r/g,'');
}
</script>