DROP USER (Transact-SQL)
Removes a user from the current database.
Users that own securables cannot be dropped from the database. Before dropping a database user that owns securables, you must first drop or transfer ownership of those securables.
The guest user cannot be dropped, but guest user can be disabled by revoking its CONNECT permission by executing REVOKE CONNECT FROM GUEST within any database other than master or tempdb.
Beginning with SQL Server 2005, the behavior of schemas changed. As a result, code that assumes that schemas are equivalent to database users may no longer return correct results. Old catalog views, including sysobjects, should not be used in a database in which any of the following DDL statements have ever been used: CREATE SCHEMA, ALTER SCHEMA, DROP SCHEMA, CREATE USER, ALTER USER, DROP USER, CREATE ROLE, ALTER ROLE, DROP ROLE, CREATE APPROLE, ALTER APPROLE, DROP APPROLE, ALTER AUTHORIZATION. In such databases you must instead use the new catalog views. The new catalog views take into account the separation of principals and schemas that was introduced in SQL Server 2005. For more information about catalog views, see Catalog Views (Transact-SQL).
The following example removes database user AbolrousHazem from the AdventureWorks2012 database.
USE AdventureWorks2012; DROP USER AbolrousHazem; GO