Prova questo
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function deleteRows(aTable) {
 len = aTable.rows.length;
 for (i=len-1;i>=2;i--) {
  aTable.deleteRow(i);
 } //  for (i=len-1;i=1;i--)

} 
//-->
</script>

</head>
<body>
<table summary="" id="t">
<tr>
<td>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</td>
</tr>
<tr>
<td>bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb</td>
</tr>
<tr>
<td>cccccccccccccccccccccccccccccccccc</td>
</tr>
<tr>
<td>dddddddddddddddddddddddddddddddddd</td>
</tr>
<tr>
<td>eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee</td>
</tr>
<tr>
<td>ffffffffffffffffffffffffffffffffff</td>
</tr>
<tr>
<td>ggggggggggggggggggggggggggggggggggg</td>
</tr>
</table>
<input type="button" value="clicca" onclick="deleteRows(document.getElementById('t'))">
</body>
</html>