Open the screensaver control panel, select Options, then turn OFF the HUD. With the HUD disabled, then the screensaver should respond to mouse movement, mouse click, or keypress by exiting.
With the HUD enabled, certain keys control the screensaver itself (Space = pause, Enter = navigation menu, Arrows = navigate). Any other key will exit the screensaver.
If not, then it's possible you are seeing a bug? What version of the screensaver are you using?