Hallo Manuela,
so gehts...
<html>
<head>
<script type="text/javascript">
function wechsel()
{
document.test.es.disabled = true;
}
</script>
</head>
<body>
<form name="test">
<input type="text" value="test" id="es">
<br><br><br><a href="#" OnClick="javascript:wechsel(); return false;">tu es</a></form>
</body>
</html>