Full screen swapping to windowed mode when I accidentally click on my second monitor, help please =P

I have a PC with dual monitors, and when I accidentally scroll off the screen and click on my second screen it drops my game window from full screen down to windowed mode (a very small window at that). I would just play in windowed mode, but the size isn't adjustable.

Is there any way to fix this issue?

I just unplug the other monitor, you cant use it anyway when the game is running