I remember with Tiger that option + F7 or F8 refreshed the connection and detected the externally connected projector, but I don’t recall a key combo that enabled disconnection. What I used to do as a workaround was toggle mirroring on/off and with mirror mode off (and setting the 2nd screen to a default black wallpaper), I was able to mimick screen blanking/muting; of course, if your projector has a mute screen button this all is moot, but I’m assuming that in your case that’s not an option?