[ubuntu] 15.10 itself changing sound output magically

Hello!

In my Ubuntu 15.10 Desktop I have 3 options to output sound:
1 - HDMI / DisplayPort 3
2 - HDMI / DisplayPort
3 - Digital Out (S/PDIF)

When I try to reproduce sound, it is always on option (1) or (3), but not (2), because output (2) is the correct one, and I always have to change it by hand. How to prevent Ubuntu magically changing without my permission the default output? If I could, I would even erase the other 2 options, as I will never use them.

Thank you!