Fijate si así te sirve:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
<script>
window.onload=function(){
setInterval(
function(){
els=document.getElementsByTagName('input')
for(i=0;i<els.length;i++){
if(els[i].type=='radio' && els[i].checked)
els[i].disabled=1;
else
els[i].disabled=0;
}
},500);
}
</script>
</head>
<body><form>
<table width="100%" border="0">
<tr>
<td><input name="var_tllamada" type="radio" value="1">
Audiencia</td>
</tr>
<tr>
<td><input name="var_tllamada" type="radio" value="2">
Invitación</td>
</tr>
<tr>
<td><input name="var_tllamada" type="radio" checked value="3">
Ninguna de las Anteriores. </td>
</tr>
</table></form>
</body>
</html>