Thanks for playing!
The popup box for building a tank can be closed by pressing the Escape key (I forgot to put that in the tutorial) or by clicking the "Cancel" button. However, if your mouse is still hovering over a factory when you press the cancel button, the popup box will immediately pop up again. I originally had a much more intuitive system, but I had some problems getting it to work in time.
Also, it's a bit ironic that you mentioned that I had a lot of levels. At the start, I had actually planned for 15 levels, but by the time the end rolled around, I had to scrap the last 5 to save time. Not that I spent that long making the levels though - making and testing all the levels probably only took about a day total.