What I did is first check what are the running processes by
SELECT * FROM pg_stat_activity WHERE state = 'active';
Find the process you want to kill, then type:
SELECT pg_cancel_backend(<pid of the process>)
This basically "starts" a request to terminate gracefully, which may be satisfied after some time, though the query comes back immediately.
If the process cannot be killed, try:
SELECT pg_terminate_backend(<pid of the process>)
