The simple answer is to turn the gains down on the input channels of the mics which are plugged in. But there's another better answer.
|Arrows and circles indicate the buttons you need to press.|
On the interface we use there are switches on input channels 1 and 2 which flip the input over to the high-impedance 1/4" front-panel jacks. So you press those switches and viola -- the microphone is turned "off".
So, that's your pro audio news from the Pandora Machine front today. You are ever so glad you check this blog.