What to do if the database principal owns a schema in the database, and cannot be dropped?

If the database principal owns a schema in the database, and cannot be dropped, try the T-SQL to do this:

alter authorization
on schema::YourSchemaName
to dbo
go
drop user TheUserYouWantToDelete
go

You can't drop a principal that is a schema owner, so the ALTER AUTHORIZATION changes the owned schema (I used YourSchemaName, but obviously substitute that with the owned schema in your database) to dbo (likewise, you can change ownership to whatever principal you need in your environment). That will allow you to drop the previously-schema-owning user (for example purposes I used TheUserYouWantToDelete, but that'll be the non-owner that you want to drop).