Eu usei o jquery-1.11.3.js
Ativar e desativar básico
Dicas 1: (Tipo de botão de opção comum Desativar e ativar)
$("input[type=radio]").attr('disabled', false);
$("input[type=radio]").attr('disabled', true);
Dicas 2: (seletor de ID Usando prop () ou attr ())
$("#paytmradio").prop("checked", true);
$("#sbiradio").prop("checked", false);
jQuery("#paytmradio").attr('checked', 'checked'); // or true this won't work
jQuery("#sbiradio").attr('checked', false);
Dicas 3: (Seletor de classe Usando prop () ou arrt ())
$(".paytm").prop("checked", true);
$(".sbi").prop("checked", false);
jQuery(".paytm").attr('checked', 'checked'); // or true
jQuery(".sbi").attr('checked', false);
OUTRAS DICAS
$("#paytmradio").is(":checked") // Checking is checked or not
$(':radio:not(:checked)').attr('disabled', true); // All not check radio button disabled
$('input[name=payment_type][value=1]').attr('checked', 'checked'); //input type via checked
$("input:checked", "#paytmradio").val() // get the checked value
index.html
<div class="col-md-6">
<label class="control-label" for="paymenttype">Payment Type <span style="color:red">*</span></label>
<div id="paymenttype" class="form-group" style="padding-top: inherit;">
<label class="radio-inline" class="form-control"><input type="radio" id="paytmradio" class="paytm" name="paymenttype" value="1" onclick="document.getElementById('paymentFrm').action='paytmTest.php';">PayTM</label>
<label class="radio-inline" class="form-control"><input type="radio" id="sbiradio" class="sbi" name="paymenttype" value="2" onclick="document.getElementById('paymentFrm').action='sbiTest.php';">SBI ePAY</label>
</div>
</div>