Xfwm4 supports showing a window in fullscreen mode by choosing Fullscreen in the window menu. One may do this using nothing but the mouse. But one can't make the window leave fullscreen using nothing but the mouse[1]. Sure, one may use a keyboard shortcut, but I believe that what one can do with the mouse, one should also be able to undo using the mouse. Besides, people doing this may not always be aware of the default keyboard shortcut. I don't really have any suggestions to a solution, besides simply removing the item from the menu, and I'm not sure that's a good idea. [1]: Unless such an option is provided by the application in the window, but that's an exception and doesn't really have much to do with Xfwm4.
Without keyboard, you'd have to assign one of the two main mouse buttons, which is really not suitable. I understand your point, but I really don't think this is suitable.