hai provato con disabled?
disabled = true/false
Per esempio:
codice:
<html>
<head>
<script language="javascript">
function manageChecks(c) {
  if (c.checked) {
    c.form.check2.disabled = false;
    c.form.check3.disabled = false;
  }
  else {
    c.form.check2.disabled = true;
    c.form.check3.disabled = true;
  }
}
</script>
</head>
<body>
<form name="form">
  <input type="checkbox" name="check1" onclick="manageChecks(this);" /> Seleziona e abilita le altre 2 

  <input type="checkbox" name="check2" disabled />Sono disabilitata

  <input type="checkbox" name="check3" disabled />Sono disabilitata

</form>
</body>
</html>