How is the computer connected to the Arc? 

I’m supposing this is the TV’s ARC turning on all connected devices, using it’s CEC connections, but would only make sense if the computer was plugged in to the TV’s HDMI port, and had CEC commands enabled, much like the Blu-ray player. My guess is if you turn off the functions on them, or even turn off CEC on the TV set, it will stop booting up every connected device when it is powered on. This shouldn’t be a Sonos Arc issue, though, just part of the way CEC is implemented on your TV. 

The Arc does not turn on in a classical sense. It powers down to a more energy efficient state and powers up when it gets the signal from your TV. If you would wake it by playing music form Spotify it should not bother the TV or devices connected to the TV. How did you arrive to the conclusion it is the Arc that powers up your TV an BD-player?

Thanks for the feedback. Turning off CEC on my LG TV also seemed to isolate the Sonos ARC unusable. I turned the CEC back on an turned off the "Auto Power Sync" which seemed to fix the issue. Thanks again for pointing me in the right direction.