Question regarding the professors attemps to stop Collosus.

Given the planning put into Colossus and Guardian it is clear the first thing you would do is give the computer own power suppply (ala the Greenbriar fallout shelter or NORAD). Also remember Colossus and Guardian were designed as information gathering system so they would notice the effort needed to cut their power. I can see tha tin a fit of paranoia we could do something this dumb.

Well, disregarding the question as to whether this scenario is realistic (I don't think it is), the problem was that the computer practically held the entire world population hostage by controlling the nuclear stockpiles of the two world powers. The bunkers housing Colossus and his Soviet counterpart were most certainly secured against nuclear strikes, and, as mentioned by the previous poster, had independent power sources that would supply them for years to come. It would be very difficult to take them out not only simultaneously, but quickly enough to prevent them from responding.

Agreed. In data centers, there is always the "big red button", which often does something catastrophic to the power supply. These are necessary to force the loss of power in case of fire. Some IBM mainframes have a sharp blade driven by a hydraulic ram that physically severs the main copper wiring.

This sort of a panic button is built into humanoid robots that are bolted to tables. Actually, many panic buttons. There's one near the door, one on the table next to the bot, one over by the console...

There was a spectacularly bad design decision made when they didn't include multiple ways of disabling the machine, including at least one dead-man switch.

Which turns out worse!

However, I think the whole "impregnable computer" scenario is all about the folly of weak men shrugging off their most serious responsibilities so they could lay it all on their machines.

"It's not my fault..."

It's another "hubris/swift and mortifying ate" scenario.