Using sys.dbms_scheduler.STOP_JOB one can stop scheduled job.

SQL> exec sys.dbms_scheduler.STOP_JOB(job_name=>'SYS.ORA$AT_OS_OPT_SY_12856', force=>true);

Output:
PL/SQL procedure successfully completed.

If it can’t find the job then you may see the following error, in the example below I hadn’t specified the user to it was looking for the job as the current user.
ORA-27475: “DBAUSER.ORA$AT_OS_OPT_SY_12856” must be a job
ORA-06512: at “SYS.DBMS_ISCHED”, line 199
ORA-06512: at “SYS.DBMS_SCHEDULER”, line 557
ORA-06512: at line 1

To see jobs running http://atomic-temporary-2592919.wpcomstaging.com/2010/05/08/dba_scheduler_running_jobs/