[PHP,Jquery] สอบถามการใช้ jquery checkbox ให้การ disable textbox

 <div class="table-responsive">
  <table class="table-bordered " id="tblImpact">
    <tr>
    <th class="cLabel col-xs-1">1</th>
    <th class="cLabel col-xs-1">2</th>
    <th class="cLabel col-xs-1">3</th>
    <th class="cLabel col-xs-1">4</th>
    <th class="cLabel col-xs-1">5</th>
    <th class="cLabel col-xs-1">6</th>
    <th class="cLabel col-xs-2">7</th>
    <th class="cLabel col-xs-4">8</th>
    </tr>
    <tr>
      <td><input type="text" class="form-control" id="txtbox1_1" name="txtbox1_1"></td>
      <td><input type="text" class="form-control" id="txtbox2_1" name="txtbox2_1"></td>
      <td><input type="text" class="form-control" id="txtbox3_1" name="txtbox3_1"></td>
      <td><input type="checkbox" id="chkbox4_1" name="chkbox4_1" class="chkB" value="1"></td>
      <td><input type="text" class="form-control" id="txtbox5_1" name="txtbox5_1"></td>
      <td><input type="text" class="form-control" id="txtbox6_1" name="txtbox6_1"></td>
      <td><input type="text" class="form-control" id="txtbox7_1" name="txtbox7_1"></td>
      <td><textarea rows="3" class="textareaInTbl" id="txtbox8_1" name="txtbox8_1"></textarea></td>
      <td><button type="button" id="addBtnImapct" class="btn btn-primary btn-xs btn-block">+</button><button type="button" id="delBtnImpact" class="btn btn-danger btn-xs btn-block">-</button></td>
    </tr>

  </table>
  </div>

 var counter1 = 2;
   
    $("#addBtnImapct").click(function () {

    $("#tblImpact").append('<tr><td><input type="text" class="form-control" id="txtbox1_' + counter1 + '" name="txtbox1_' + counter1 + '"></td><td><input type="text" class="form-control" id="txtbox2_' + counter1 + '" name="txtbox2_' + counter1 + '"></td><td><input type="text" class="form-control" id="txtbox3_' + counter1 + '" name="txtbox3_' + counter1 + '"></td><td><input type="checkbox" id="chkbox4_' + counter1 + '" name="chkbox4_' + counter1 + '" class="chkB" value="1"></td><td><input type="text" class="form-control" id="txtbox5_' + counter1 + '" name="txtbox5_' + counter1 + '"></td><td><input type="text" class="form-control col-xs-1" id="txtbox6_' + counter1 + '" name="txtbox6_' + counter1 + '"></td><td><input type="text" class="form-control col-xs-2" id="txtbox7_' + counter1 + '" name="txtbox7_' + counter1 + '"></td><td><textarea rows="3" class="textareaInTbl" id="txtbox8_' + counter1 + '" name="txtbox8_' + counter1 + '"></textarea></td><td></td></tr>');

    counter1++;
  });
     
    $("#delBtnImpact").click(function(){
       if(counter1==2){
    alert("ลบไม่ได้ละครัชชช");
    return false;
    }

     counter1--;
     $('#tblImpact tr:last').remove();
    });