You will need to update the FND_CONCURRENT_REQUESTS table to cancel the request from the backend.
Please refer the following links, as it has detailed explanation:
In addition to the above, for further readings also Let me google that for you
NOTE: Dont use alter system kill session command to terminate concurrent request, as the EBS behaves different to standalone DB
Hope this helps,
Concurrent Processing - What are the Meaning of the Codes in the STATUS_CODE and PHASE_CODE Columns of FND_CONCURRENT_REQUESTS Table? (Doc ID 152209.1)
Concurrent Processing - How to Find Database Session & Process Associated with a Concurrent Program Which is Currently Running. (Doc ID 735119.1)
OERR: ORA 27 cannot kill current session (Doc ID 18232.1)