The first step will be to disable Factory Reset Protection (FRP). In this way, in case of theft or loss, it will prevent the deletion of all the data present and the restoration of the factory settings. Basically, this feature makes it more difficult to use a stolen phone. From the Settings menu, click on Screen lock, Screen lock type and select None.
You can now proceed with the removal of your account. From the Settings menu you will have to select Accounts and backups, Managed accounts and click on the one you intend to remove, confirming with Remove account.
Third step: factory reset the phone. Resetting the device will delete all the data present. Again from Settings you will have to select General management, Reset and select Factory data reset. Before confirming a window will appear with a list of all data, accounts and apps that will be removed with this operation. By clicking on Reset and entering the account password, the deletion will begin.