@Freithelm Fabian
Yes agree can see anything obvious in the Fritz Box that could help. If only the parental controls was extended to LAN ports 
I found this useful article:
https://scottiestech.info/2015/05/14/how-to-block-apple-bonjour-on-your-local-network/
However what I suggested was always going to a quick work around if your router had something that way. It is certainly not a generic solution.
The more I think about it the best way is as you suggest have the capability to disable the service. I used to use Volumio on RPI and in the web interface I could enable and disable DLNA/UPNP or Airplay.
@zpl1025 is it easy/possible to disable Airplay & DLNA Renderer Service on the A31 boards ?