You may try to use
rm -rf [directory] to delete given directory along with all its content (including all subdirectiories and all files). Use this with caution, to not end up deleting all files on your root! :>
Note, that on QNAP (but not on regular Linux) you have to start your session as admin user, as you cannot use
su command to become root. If you try this, you'll end up with error
su: user root does not exist.