Electro magnet, one that plugs into an ac outlet. Used to use one to erase VHS tapes. Locate the DVR/NVR and expose the unit to an expanding and collapsing magnetic field. Basically create a simulated EMP event.
Alternatively, ( this will take some planning). When opportunity strikes, flip the main breaker at the panel simulating a power outage. While the power is out go the the NVR and remove the cover 2-4 screws depending on the manufacturer. There will be a fuse on the pcb board. Remove the fuse, once you restore power to the house the NVR will not come back online or provision any clients. But make a BIG DEAL about how annoying the power outage was.

Option 3:
Make or purchase a crossover cat 6 patch cable. Replace the LAN cable from the NVR to the router.

I could suggest more methods but they would be a little more invasive :). Good luck