I don't think it's necessary to physically cut cables.
What if critical infrastructure is isolated from savages by just not having routes to/from them?
I mean, in a similar way the Team Cymru UTRS[1] works by publishing a list of BGP filters to be imported in routers.
Probably I'm just naive but I really don't understand this cyber warfare theater. Why aren't all internet operators in the free world just blackholing all russian ASNs (complete with the other allied bullies) from the internet routing tables? Wouldn't this be enough to avert all the nasty attacks on critical infrastructure?