DROP LOGIN (Transact-SQL)
Removes a SQL Server login account.
DROP LOGIN login_name
Specifies the name of the login to be dropped.
A login cannot be dropped while it is logged in. A login that owns any securable, server-level object, or SQL Server Agent job cannot be dropped.
You can drop a login to which database users are mapped; however, this will create orphaned users. For more information, see Troubleshooting Orphaned Users.
Requires ALTER ANY LOGIN permission on the server.
The following example drops the login
DROP LOGIN WilliJo; GO