In the 975 speaker setup menu simply select "none" for the center channel. The 975 will handle the rest automatically.

There is a "stereo" button on the remote but you need not concern yourself with it in the situation you have described. Just set the distances and levels for the speakers you are running and forget about the rest.

P.S. I just checked on my 975 to verify that while you can run in the "stereo" mode if you wish to change the "surround mode" option the 975 will automatically drop back out of "stereo" mode.