Joe, there is a dedicated thread specific for AP questions.  Please look for it.

At this time, that is not possible, and the reason as best as I am aware is because FS considers deselection of CMD status to be the same as AP Disconnect.  However, a real world boeing does not work this way.  Once AP CMD is selected, you have two options- 1) select another AP to be in command, or 2) pull out the AP Disengage bar.  Since there is no command for "AP Disengage Bar" there is no way within FS itself to set this.

However, it may be possible via FSUIPC?  I don't know this for sure.  I prefer not to touch my keyboard at all during flight, and this is facilitated via yoke, throttles, buttons on those devices, etc.  I do have a button on my yoke assigned to AP Disconnect, but of course this doesn't work in the CS 757.  I think the real plane has a button which WILL "pull" that bar up with a magnetic release, but instead if I need to disengage (I.E. visual approach) I will reach up to the MCP and pull the bar with the mouse click.

Hope it helps.