Stop neverending backup SQL Server Express 2012

Our production database log file has grown to consume all disk space on our server.

I know it's bad practice but I want to be able to shrink it in order to restore the service as soon as possible.

Still, when I go and launch my SHRINKFILE command,I get this error :

Msg 3023, Level 16, State 2, Line 2

Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed.

It seems to me that the backup will never end. Is there a way to stop it manually ?

(I tried the SQL Server Agent thing, as told in the msdn documentation, but it is not present in my SQL Server Express)