With the recent changes to the way the discovery component works, I am no longer able to stop it from seeing my google_cast devices for example. I know others have issues with homekit devices being found and if you choose not to set it up or use it, the ‘dismiss’ option, you get a notification next time HA loads up!
Now it would be nice if somehow we could add google_cast or homekit to some sort of blacklist, similar to blacklisting linux kernel modules here While this may not look simple, HA could so something more simple with the YAML files I’m sure.
I know it’s cool that HA is able to find and use so many things on the LAN, but not everyone wants everything to be found and used, some finer grain control would be nice. 
If that is not an option, is it possible to block certain devices from seeing HA or beeing seen by HA on a network level? Like some sort of built in firewall? I know HassIO is essentially docker, and some docker wizard maybe able to do this, but it would nice if this was possible in the frontend somehow, that mere mortals can use! Or if HA had it then you wouldn’t need HassIO then, but I’m not really sure if this is or isn’t possible.