Stop never ending backup in SQL Server 2012 Express
Restarting SQL Service will kill backup process and then you can shrink. Below query will show you spid of backup process, if any running on server.

SELECT *
       FROM master.dbo.sysprocesses
       WHERE --dbid = db_id('YourDatabase') AND --if you want to see status for database
       cmd LIKE 'BACKUP DATABASE%'


Below query will kill backup process if you don't want to restart SQL service.

Declare @SPIDU Varchar(10), @Query Varchar(50);
SELECT top 1 @SPIDU = spid
       FROM master.dbo.sysprocesses
       WHERE --dbid = db_id('YourDatabase') AND --if you want to see status for database
       cmd LIKE 'BACKUP DATABASE%'
Select @Query = 'KILL ' + @SPIDU + ';'
Exec( @Query)


Make sure you do clear trans log by taking backup or else you wont be able to reclaim space from trans log file.