How to make macbook output to external monitor only

[Combat squirrel]

As title, im trying to get my macbook to output to my 2407 ONLY when the DVI cable is attached, currently it only seems to let me either mirror or span, I cant seem to see the option for this (force output to one monitor only, i.e. the external one), I maybe being daft! lol but any help is appreciated

 

[Combat squirrel]

What? turning the brightness down forces one monitor to display on external cable ? Really ? thats bizarre,lol

 

[Combat squirrel]

Well yeah but why on earth apple cant add an option to just output to one monitor only is beyond me.