I do manage a CCTV Surveillance site as well and the only way to delete events from the logs is to log-in directly to the interface of the NVR. You can't do it remotely using the client or the web interface. The administrator user needs to connect to the administration interface, usually with a keyboard and an HDMI cable (depending on the output mode available for the CCTV interface) and then access the logs.
I believe this is explained very well in the user guide or manual (the files are stored in the installation directory).
