I don't believe there is such a thing as a "channel lock". "Volume Punch-Thru" is indeed a special feature that allows the Volume of one device to appear automatically on some of the other devices. But there is no dedicated Channel Lock feature on the Cinema 7 as far as I know.
However, you can use "Key Mover" to do what you want with the Channel up/down buttons. Just "move" the cable receiver Channel Up/Down buttons, one at a time, to whatever other device(s) you want them to appear. You can read all about Key Mover, and all the other built-in programming functions of the Cinema 7, at
[Link: hifi-remote.com]. Good luck!