rat,
With most 7 channel amps, there is no way of turning off the amps that are not in use, and it is really no big deal.

With receivers and pre/pros, when setting up the speakers, you tell the system if it has 2, 3, 4, 5, 6, or 7 speaker channels in use, and it sends the appropriate signals to each. Some even allow for channels 6 and 7 to be used in a different "zone", which provides 2 channel information in to another room.