When the NVR is in PnP (Plug and Play) mode, it will not allow users to delete cameras that was automatically added when connected to the PoE ports from the back of the NVR.
To resolve this issue, go to Setup -> System -> System management -> Settings and uncheck "Enable PnP mode" then Apply.
After applying the setting, if you go back to Setup -> Camera -> Channel setup, and you should be able to delete the previously greyed out channels.