有人给我留下了正确的答案然后因某种原因将其删除了，所以这里是：

Someone left me the correct answer and then removed it for some reason, so here it is:

function IgnoreAlpha(e) {
  if (!e) {
    e = window.event;
  }
  if (e.keyCode >= 65 && e.keyCode <= 90) // A to Z
  {
    e.returnValue = false;
    e.cancel = true;
  }
}

<p>
  <select id="MySelect" name="MySelect" onkeydown="IgnoreAlpha(event);">
    <option selected="selected " value=" " />
    <option value="A ">A</option>
    <option value="B ">B</option>
    <option value="C ">C</option>
  </select>
</p>