We can kill RMAN backup job with the help of 2 methods, as below:

Method I:
Alter system kill session:

First, get the SID and SERIAL# from below query:

SQL> select b.sid, b.serial#, a.spid, b.client_info
from v$process a, v$session b
where a.addr=b.paddr and client_info like 'rman%';
SID        SERIAL#    SPID         CLIENT_INFO
---------- ---------- ------------ ---------------------------------
592        12         865          rman channel=full_chanel

OR

SQL> SELECT SID, SERIAL#, CONTEXT, SOFAR, TOTALWORK,
ROUND (SOFAR/TOTALWORK*100, 2) "% COMPLETE"
FROM V$SESSION_LONGOPS
WHERE OPNAME LIKE 'RMAN%' AND OPNAME NOT LIKE '%aggregate%'
AND TOTALWORK! = 0 AND SOFAR <> TOTALWORK;
SID        SERIAL#    CONTEXT    SOFAR      TOTALWORK  %COMPLETE
---------- ---------- ---------- ---------- ---------- ----------
592        12         1          9115569    19258880   47.33

Use the following command to kill RMAN backup job:

SQL> alter system kill session '592,12' immediate;
system altered.

Backup job killed successfully, simultaneously you will get below mentioned error log in RMAN backup logs:

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup plus archivelog command at 07/01/2017 00:09:15
RMAN-10038: database session for channel full_chanel terminated unexpectedly

Method II:

Directly kill RMAN job from OS level with the help of “kill -9”

[oracle@PR ~]$ ps -ef | grep rman|grep -v grep
oracle 2348 3124 3 01:28 pts/1 00:00:00 rman target /

[oracle@PR ~]$ kill -9 2348

Thanks, Stay Tune. 🙂