By default in "config_defaults_inc.php", it is allowed to modify (Edit), delete and make private a bugnote.

I have made this changes:

$g_private_bugnote_threshold = DEVELOPER; -> ADMINISTRATOR
$g_update_bugnote_threshold = DEVELOPER; -> ADMINISTRATOR
$g_delete_bugnote_threshold = '%delete_bug_threshold%'; -> ADMINISTRATOR
$g_delete_bug_threshold = DEVELOPER; -> ADMINISTRATOR
$g_bugnote_allow_user_edit_delete = ON; -> OFF

For a developer, it is OK but a manager have two choices for a bugnote : Edit and Delete (see file with this bug), even if this bugnote was not write by him.