Pull the power cord. What are you actually trying to do?

There’s no way to ‘delete’ a speaker from your system. Sonos remembers it for a while before it silently disappears from your system list when you login to Sonos.com 

So, the best way to ‘delete’ it would be to factory reset it, so it won’t rejoin your system and pull the plug. 

I don’t know if Sonos Support can delete speakers, but I think they’re a bit busier than normal at the moment. 

It doesn’t seem to prevent you from adding it to a new system, so should be safe to move on if that’s your intention.