When you registered the taxonomy with register_taxonomy there is a capabilities argument. This takes, an array of the capabilities for this taxonomy. In particular:
- ‘manage_terms’
- ‘edit_terms’
- ‘delete_terms’
- ‘assign_terms’
Associated with each of them should be a capability that it is required to be able to perform that action. For instance to assign_terms usually requires the user to have the capabiliy of edit_posts. You can give the first three some capability (custom or otherwise, manage_options might do) that your clients do not have, but you do. The last can be just edit_posts.
As an example:
register_taxonomy('mytax',array('post'), array(
'hierarchical' => false,
//Other properties...
'capabilities'=>array(
'manage_terms' => 'manage_options',//or some other capability your clients don't have
'edit_terms' => 'manage_options',
'delete_terms' => 'manage_options',
'assign_terms' =>'edit_posts'),
//Other properties...
));