macOS/iOS devices reconnect automatically to airplay/bluetooth device disturbing intended audio stream
It may sound like a simple solution, and it may not be what you're looking for, but it is the easiest to accomplish: turn off Bluetooth on the devices you don't want to connect.

Personally, I don't like leaving BT on and running on my devices when it is not in use. A quick swipe on the Control Center and the On/Off toggle is right there.