Hi 
As you are using the NAS functionality of the QNAP the file deletion and management will have to be managed at the Camera side.  If you had used a QNAP VioStor and used the camera function that is built into the VioStor the QNAP could have managed the file deletion. 
  
http://data-storage.co.uk/Network-Video-Recorders-NVR/qnap-security.html 
  
I hope this helps.