Prevent automatic Target Display Mode on 27" iMac

Hey everyone,

I've got a 27" iMac and an 11" MacBook Air.

I use the iMac as a display for the MBA through target display mode. I used to have the MBA open to make use of its trackpad and keyboard, but I just recently bought an additional Apple Bluetooth Keyboard and Magic Trackpad so that I can use the MBA closed whilst the iMac is in target display mode.

When I want to switch back to the iMac, I just press cmd+F2 on the iMac's keyboard to exit target display mode.

The problem I'm facing is that the MBA, whilst still plugged into the iMac, but with target display mode off, seems to go though some sort of cycle where it tries to regain the iMac's display. Every so often, whilst using the iMac, I have to press cmd+F2 when the MBA tries to steal the display.

The obvious answer is to unplug the MBA, but I switch between the two very often and it makes more sense to just press cmd+F2 when I'm ready to toggle between the machines.

Does anyone know how to disable automatic target display mode on the iMac? I'd like it so that target display mode won't be allowed unless I've pressed cmd+F2 on the iMac's keyboard rather than any time a signal is detected on the display port cable.