How to disable multiple cardinality for a custom field
Just alter the cardinality selection with this hook and provide only one value:

function &lt;my_module&gt;_form_field_ui_field_edit_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form['#field']['type'] == '&lt;my_field_type&gt;') {
    $form['field']['cardinality']['#options'] = array(1);
  }
}