Hi Dear
please try this
function DisableRadioButton() {
var e = document.getElementById('<%=DropDownList1.ClientID %>');
var strUser = e.options[e.selectedIndex].text;
if (strUser == "unauthorized") {
document.getElementById('<%=RadioButton1.ClientID %>').disabled = true;
document.getElementById('<%=RadioButton2.ClientID %>').disabled = true;
}
}
add this line in html onChange="DisableRadioButton();"