How to disable 1 of my 2 monitors?

Hi,

First, I need to say that, regarding my problem, I wasn't sure if I should post in this section or in the 'MacOSX' one, but like I'm facing this problem on my Mac Pro 4,1...

So, I have a 8-core Mac Pro 4,1 with an Apple HD4870 to which are connected 2 screens:
the main one is a Dell U2410 (1920x1200, via mDP to DP adapter) that I use under OSX for photography retouching with DxO.
The other one is a 27'' (1920x1080, DVI) I only use for gaming under bootcamp.

When under bootcamp, I only need to turn off the Dell and the desktop is resized for only 1 display.

My problem is that, under OsX, even when the 27'' is turned off, the desktop is extended to this display, I can't find a way to deactivate the 27''.
I can't even set the displays as 'mirrors', as they have different heigts in pixels (1200 vs 1080), and then I get a Dell resized to 1080

The only solution I have right now is to disconnect the DVI cable of the 27'' when using the computer under OsX, which is %&*)(@#$ (see what I mean)

Anybody ever had this problem, or has any clue about how to deactivate one of the 2 displays under OsX?

Thanks for your replies, and apologies if I posted in the wrong section

 

Thanks Hellhammer, I'll try that.