I'm on drupal 7.38 and
to only allow input from select-box in autocomplete
you only need to delete the user-input at the point,
where js does not need it any more - which is the case,
as soon as the search-results arrive in the suggestion-popup
right there you can savely set: 
 **this.input.value = ''**
see below in the extract from autocomplete.js ... 
So I copied the whole Drupal.jsAC.prototype.found object
into my custom module and added it to the desired form 
with 
 $form['#attached']['js'][] = array(
  'type' => 'file',
  'data' => 'sites/all/modules/<modulname>_autocomplete.js',
 );
And here's the extract from drupal's original misc/autocomplete.js
modified by that single line... 
Drupal.jsAC.prototype.found = function (matches) {
  // If no value in the textfield, do not show the popup.
  if (!this.input.value.length) {
    return false;
  }
  // === just added one single line below ===
  this.input.value = '';
  // Prepare matches.
=cut. . . . . .