Can't delete a login due to db ownership — but no dbs are owned by this login
You should check the ownwers of the databases with the following script:

SELECT d.name, sl.name
    FROM sys.databases as d JOIN sys.syslogins AS sl 
        ON d.owner_sid = sl.[sid] 


You might then find out that a database is linked to on of your Windows/AD accounts.

There is a slight difference between a SQL Login (native or Windows account) having the role db_owner and an account being the database owner.

If you do find a database with an account linked to the Windows/AD account you want to get rid of, then execute the following command while connected to that database:

USE &lt;databasename&gt;
GO
sp_changedbowner '&lt;new_account&gt;'



  IMHO: Changing the owner to 'sa' is a viable option.


Good luck.