Ok, I signed out of the unwanted IMDB account on that site. Then I checked whether I was still signed in on Amazon and found I had been signed out of Amazon. So that seems like a bad sign for what "Delete my account" means on the IMDB site. And they are not clear about it. That's horrible. There seems to be no way now to be signed in on Amazon and be signed out on IMDB. What a steaming pile of shit.