First hook the save_post action: http://codex.wordpress.org/Plugin_API/Action_Reference/save_post
Then check capabilities: You have docs on that right?
Then get_the_terms for the $post_id provided by save_post and the taxonomy you want to check. http://codex.wordpress.org/Function_Reference/get_the_terms
Then if they are using a term (see note below) they are not allowed to use either stop/delete the post, set a warning, or remove the term.
Note: a taxonomy is a list of terms that can be applied to a post. Example: the taxonomy “Nation” would contain terms like “Mexico”, “France”, and “India”