Sabotage
Sabotage
This approach seems impractical. If the goal is to make the damage undetectable, using a static IP on another network would be a better solution. The local internet connection wouldn't function properly.
With access to his laptop, you can turn off the Ethernet adapter. (Just right-click to enable or disable in: Control Panel\Network and Internet\Network Connections)
You believe using cable cutters on the Ethernet cable and replacing it is sufficient. The cost is only three dollars.
Updated the admin credentials for the router. Added a mac filter to the user's PC/laptop.