How to modify a term reference autocomplete field
There's a GREAT module for this type of configurations called Hierarchical Select


  Hierarchical Select has the ability to save the entire lineage of a
  selection or only the "deepest" selection. You can configure it to
  force the user to make a selection as deep as possible in the tree, or
  allow the user to select an item anywhere in the tree. Levels can be
  labeled, you can configure limit the number of items that can be
  selected, configure a title for the dropbox, choose a site-wide
  animation delay, and so on. You can even create new items and levels
  through Hierarchical Select!


This module has a lot to offer, but as for your need, it also does the trick.
Also you might want to consider Taxonomy Term Reference Tree Widget as well, the main functionality is a different thing but you can define a permission via views to do what you are after too with this module.