– how to allow only one item selected?

Just don’t make it a select multiple, but set a size to it, such as:

  <select name="user" id="userID" size="3">
    <option>John</option>
    <option>Paul</option>
    <option>Ringo</option>
    <option>George</option>
  </select>

Working example:
https://jsfiddle.net/q2vo8nge/