In SQL Server 2005, if you try to drop a database user by right-clicking the user name and clicking Delete, you may encounter an error as follows.
"The database principal owns a schema in the database, and cannot be dropped. (Microsoft SQL Server, Error: 15138).
This problem normally occurs when a user owns a schema and you are trying to delete the user.
There are many workarounds for these one is
Using the GUI in "Object Explorer" (MS SQL Management Studio) Expand the [databasename] / Security.
Click on Schemas.
In summary window, determine which Schema(s) are owned by the user and either change the owner or remove the Scheme(s).
The user can then be deleted.
Or using AdventureWorks DB you can for example:-
USE AdventureWorks;
GO
ALTER SCHEMA HumanResources TRANSFER Person.Address;
Go
And HAPPY GO LUCKY