Use sp_dropalias to drop the mapping of an alternate suid to a user ID. Doing this deletes the relevant row from sysalternates. The syntax is the following, where loginame is the name of the user specified by loginame when the name was mapped with sp_addalias:
sp_dropalias loginame
After a user’s alias is dropped, the user no longer has access to the database.
You cannot drop an alias for a user who owns objects in the database that were created with version 12.0 or later. You must drop the objects (re-creating them under a different login, if needed) before you can drop the alias.
