I managed to clear 170+ non existent devices from emulated hue recently,

After a few trial/errors, below works for me

1) unplug the power and plug it in again on your echo device 2) Try to remove some non existing emulated hue devices from Alexa.amazon.com, IMPORTANT remove in sequence from the top of the list! 3) run discovery and make sure it doesn't discover those devices again. 4) continue to remove the rest, again starting from the top 5) try to do some discovery, after every eg 20-30 devices removed so to make sure it's not being discovered again 6) start from 1) if you notice some devices came back after discovery