Using simple (text) API functions, there is no easy way of removing all items at once (one has to loop through the items and remove one at a time using gtk_combo_box_remove_text).

In order to be useful, I wrote a function for this (I haven't created a patch, since this is a simple task of inserting function in the right spot inside gtkcombobox.c file - the right spot being maintainer dependent - and modifying corresponding header file).

-- CODE --
/**
 * gtk_combo_box_clear_text:
 * @combo_box: A #GtkComboBox constructed using gtk_combo_box_new_text()
 *
 * Clears all text strings stored in @combo_box. You can only use this
 * function with combo boxes constructed with gtk_combo_box_new_text().
 */
void
gtk_combo_box_clear_text (GtkComboBox *combo_box)
{
  GtkListStore *store;

  g_return_if_fail (GTK_IS_COMBO_BOX (combo_box));
  g_return_if_fail (GTK_IS_LIST_STORE (combo_box->priv->model));

  /* Is next test needed? */
  g_return_if_fail (gtk_tree_model_get_type (combo_box->priv->model, 0)
		    == G_TYPE_STRING);
  
  gtk_list_store_clear (GTK_LIST_STORE (combo_box->priv->model));
}
-- /CODE --