[Ned Scott]

This might work, but I'm not entirely sure. It should have the same effect as what Memphiz suggested above, but would only lock the screen selection:

place this an an advancedsettings.xml (wiki) file in the userdata (wiki) folder:

Code:

<advancedsettings>
    <videoscreen>
        <screen>1</screen>
    </videoscreen>
</advancedsettings>

and I think:
0 = first screen
1 = second screen
2 = third screen
etc