How to delete Google account from a factory-resetted phone? I have an Android 5.x smartphone ("phone 1"), which was using my Google account. Recently, I've started using different one, so I've (pretty confident that I've) dropped Google account on "phone 1", and then after having all things set up on "phone 2", I've made a factory reset on "phone 1"

However, when my son is now trying to begin using "phone 1", the initialization procedure REQUIRES him to use the account that was used on this phone before !! I presume this is good for protection against thieves, but how can I allow him use this phone ? I don't want to login under my account on that phone again
(I've done reset via some hardware menu, it had such items as wipe device, etc.)

After asking the question:

I've logged into Google Accounts as that user on a desktop, and removed that device from a list of "my devices"

It did not change anything. 

Then I've logged on THE DEVICE as that user. Then I was provided an option to login as a different Google account, as I did. So that solved my problem.