"Alter Database" usually works. However, it may fail if there are un-committed distributed transactions. In this case the SPID is not showing under the database, and can't be terminated by "Alter Database"

I usually would run the "Kill" script I posted here to "clean up" ALL, explicit and implicit, connections. Then Run the "Alter Database Set Single_User..." to claim the exclusive connection right to the database in order to do my work.

I have seen DBAs got stuck in this situation, and ended up going for the last resort: restart SQL.

Hope the above has explained.