Terminate rman hang process at Unix OS leve

To do this you have to get the process id first:
SELECT p.SPID ,CLIENT_INFO
FROM V$SESSION_WAIT sw, V$SESSION s, V$PROCESS p
WHERE sw.EVENT LIKE 'sbt%'
       AND s.SID=sw.SID
       AND s.PADDR=p.ADDR;

SPID EVENT        SEC_WAIT STATE                CLIENT_INFO
---- ---------- ---------- -------------------- -------------
8642 sbtwrite2         600 WAITING              rman channel=ORA_SBT_TAPE_1
8374 sbtwrite2         600 WAITING              rman channel=ORA_SBT_TAPE_2

Now we will kill the processes by:

kill -9 8642 8374