Remove multiple users from custom user set

Of course there is a way to do this with script. The answer to every question of the type "is it possible?" is YES if you use script.

You modify the <select> to add a multiple attribute so multiple users can be selected and then further modify the RemoveUser() function (original shown below) to have the behavior you want.

function RemoveUser(id) {
  var select = document.getElementById(id);
  if (select.selectedIndex == -1) {
    showError("You must first select a name to remove from the list.");
    return
  }
  if (!IsExplanatoryOption(select.options[select.selectedIndex])) {
    var saveIndex = select.selectedIndex;
    select.options[select.selectedIndex] = null;
    if (saveIndex < select.options.length) {
      select.selectedIndex = saveIndex
    }
  }
}