Click the 
button in the upper right corner of the title
bar to close the application
Click the 
button in the upper right corner of the menu bar to close the active
document
Click the application/document control button on the left side of the title bar and select Close
Press ALT+SPACEBAR+C (Voxler application window) or ALT+HYPHEN+C (document in the Viewer window)
Press ALT+F4 (Voxler application window) or CTRL+F4 (document in the Viewer window)