Yep. Do you have any idea how big a 100+A relay is? (~1 cm contacts) The cooling? And you'd need a duplex for std N.American service (230V hot-hotinv). Look at a 50A AC relay. Smaller & fewer for UK/EU. But meters are buss-bar straddle devices. You have to physically pull the meter out of it's socket to cut power.
Now a malefactor certainly could interfere with the power usage signals, and potentially confuse higher (optimizing) layers of the grid load-balancing system. Even that should not result in a brownout, let alone a blackout.