Microsoft SQL Azure database
Can not delete a user in the database from SSMS.
The database principal owns a schema in the database, and cannot be dropped. (Microsoft SQL Server, Error: 15138)
Go to inside SSMS, check what schema is with this command (replace donald with the login you want to check):
select * from information_schema.schemata where schema_owner = 'donald'
Change the schema stage to use DBO instead with command:
ALTER AUTHORIZATION ON SCHEMA::[Stage] TO [dbo]
After the schema is changed to have a different owner, then the user can be dropped in SSMS.
To list all user in database enter:
SELECT DB_NAME(DB_ID()) as DatabaseName, * FROM sys.sysusers