How to drop database that I can't get out of single user mode?
Stopping the server and disabling the sa account seems like overkill to drop a database. Next time this happens, the following should be sufficient:

USE [master];
GO

ALTER DATABASE [dbname] SET OFFLINE
    WITH ROLLBACK IMMEDIATE;
GO

DROP DATABASE [dbname];


The connection that could be sneaking in could be from anything - SQL Server Agent, a linked server, IntelliSense in your query window, etc. Setting the database offline is much more direct approach than even moving into single user mode, never mind unnecessarily tracking down some rogue session in deprecated compatibility views...