Delete from where? Your controller? Or the Sonos website listing of products you’ve registered to your account? Or are you just trying to delete the alarm?

I guess to achieve the result I want, I can delete the alarm or delete the product. But the only delete options I have are my current system/network/products. Can I delete this “missing” product from its own system without forgetting/deleting every speaker and my current system? 

I’m not sure if it works while there’s an alarm, but if you go to Settings > System > (Room name of missing speaker), there should be a ‘Hide This Product’ button. 

Unfortunately because the system for this specific speaker was in a short term rental on a different router than my “home” system, it doesn’t exist in the app at all, except every morning when the pop up shows “Sonos roam 7:00am Alarm” to remind me I once had it. 

Then I’m not sure there is much I can think of, other than suggesting you call Sonos Support directly to discuss it.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.