I am trying to delete an aliased user from a sql server 2000 database that was restored to sql server 2008 r2. Originally, I was attempting to convert the aliased user to a regular user, but it does not appear that I can do that, so now I am attempting to delete the user without deleting the login.
You can use
sp_dropalias do drop the legacy SQL Server 2000 user alias in SQL Server 2008 R2:
EXEC sp_dropalias N'<name-of-aliased-login-here>';
This stored procedure was removed in SQL Server 2012 later versions.