When QuizXpress Live is running in full screen mode you can close the program as follows:
Use the standard Windows keyboard shortcut Alt+F4
Move the mouse to the top-right screen position of the quiz player until the Close button appears and click it
Open the QuizXpress Director screen (press the D key in your quiz player) and locate the Exit button on the left sided command panel and click it
Get out of full screen mode by pressing the M key (maximize/minimize toggle) and click the standard Windows close button [X] on top right of the quiz player window