Hi, a Delete button will appear after you press the device a little long time on the Kasa.

Yes, you can delete it that way and it just shows back up in your list the next time you open the app. AND, I am not using it. It is unplugged, was put back in the box and mailed off to my sister. I simply tried it out to make sure it would work before I mailed it to her. Then I hit the reset button AND deleted it from my app, but it keeps coming back!!! It's frustrating as heck because I want to use the name for another one of my plugs, but if I do, alexa and siri keep telling me i have 2 devices with that name and to go rename them, and I look, and sure enough, the damn thing is back in my list again.

What's the firmware version of this plug which can't be deleted? I heard that old firmware has a bug which unable to unbind kasa account through reset.

This is a definite bug in KASA services. You'll see this all over the net people asking about Google Home and Amazon Alexa - when it's really Tp-link/kasa issue and them not wanting to fix it.
Luckily I had my old device still around so I had to power it on before the app would let me edit it. Then I gave it a new name - unused switch. This was the only way to get it out of the way of everything else.
Very bad on you TP-LINK/Kasa. This is all their cloud service needing a LOT of work!