VirtualBox machines usually have two bars in their window to make working with them easier.
These however are a great disturbance if you want to take screenshots of your machines - even if you select to leave the window border, the menu bars will still be on them.
To remove the bars, it only needs a simple terminal command. Open one and enter following (make sure that no virtual machines are running):
/usr/bin/VBoxManage setextradata global GUI/Customizations noMenuBar,noStatusBar
Next time you start a VirtualBox vm, there won't be any menu bars. To restore them again, enter following into a terminal:
/usr/bin/VBoxManage setextradata global GUI/Customizations MenuBar,StatusBar