How to disable "always on" USB power on Macbook Pro
This seems not to be possible. I solved the situation now with the help of my dell external displyay. The USB-Hub there can be configured to shut down as soon as the display ist switched off. So the second display and the active speakers are connected to the power plug that can be switched off/on by USB power. This will allow with the klick of one button to switch all other devices off/on.