[Satan]

Mufti did cut MARC II off. But I guess MARC II can force unblock itself? At least into a LAN connection.
The 'tool to wipe out humanity' is MARC II itself. It converted itself into a virus which he will upload and enables it to control every single piece of technology in the world. But since it can't even reach the Internet I suppose the film won't reveal that things will crash once the virus started lol. Forgive me for not explaining this one.

Cutting off from the internet, and cutting off from the network are two different things.
Change WiFi pass, boot off Marc 2, block connections from that MAC address. Enable Marc 1's control, use it to turn away power from whatever outlet Marc 2 is plugged into.

You're making this kinda unrealistic.