How do I remove Apple Account/iCloud from a stolen iPhone?

Here is what to do: If your iPhone or iPad was stolen - Apple Support

You may get a call or text from “police” or “Apple” requesting you prove ownership of a found device by entering your Apple Account credentials. Don’t. That’s the thief.

If you remove your iPhone from the list of devices associated with your Apple ID, you will be letting the thief or fence reset your phone to “make your phone their own.”