The CM-7004 lock function is to make a channel 'unavailable'. You wont be able to use that channel until you 'UnLock' that channel by using a 4 digit PIN that you have created.
To Lock out a channel; Press Menu on your remote control. Use the navigation buttons to highlight 'Lock'. Now press the OK/Select button. Press the Green button, and you will see the list of the channels that were scanned in. Navigate down to the channel that you want to Lock out. Press the OK/Select button. It will now ask you to enter your 4 digit PIN. (If if this is the first time you are 'Locking' a channel, it will ask you to create a PIN, then confirm the PIN.)
Once you have entered your PIN, highlight the channel that you want to Lock. Press the OK/Select button. You will now see an icon of a 'red padlock' to the far right of the channel number. Now when you open the guide, that channel info/titles, will not be available. That row will be empty. You will see "LOCK" where the channel number used to be.