It would be handy in some cases to be able to purge power from a machine.

For example, the electric pump will start immediately pumping when moved from one location to another, with no way of stopping it.

This is annoying if you are setting up a pump array in the nether and the source lava blocks are disappearing for adjacent blocks before you get a chance to place another pump.

I had have to get them to pump water into a drum until they run out of power.

While on the subject of the electric pump, it also needs a dump button for the liquid stored.

Are electric pumps not redstone-controllable? If not that's an oversight on our part, turning it off that way would be the recommended way of doing what you're having trouble with.

It does not seem to be reacting to redstone signals:

I screwed up the res and don't want to rerecord, but it shows it well enough:
http://youtu.be/zYFCiFKhgYE

Seems we didn't have the pump implementing IRedstoneControl. That's now rectified. Hope it helps.