Which method can physically stop a device from accessing a network?

Physically stopping a device from accessing a network can effectively be achieved by disconnecting the Ethernet cable. This action removes the device's physical connection to the network medium, preventing it from sending or receiving data. Unlike other methods listed, which may rely on configurations or software implementations, disconnecting the cable is a straightforward, tangible action that ensures no communication occurs.

Router reconfiguration involves changing settings within the router to control traffic or access, but this does not physically sever the connection. A software firewall setup provides a barrier to malicious traffic but can still be bypassed if the device maintains a physical connection. Utilizing a Virtual Private Network (VPN) creates a secure tunnel for data transmission over a network but does not prevent access to the network itself; rather, it enhances the security of the data that travels across it. Hence, physically disconnecting the Ethernet cable is the most direct method to halt network access.