To get a list of running SQL queries, log into the PostgreSQL CLI:

psql -U username database

Replace username and database with the proper values. Then run this query:

SELECT procpid,datname,usename,client_addr,waiting,query_start,current_query FROM pg_stat_activity;

Inside the list is also the procpid, which you will need to kill a running query. To kill, run this query, substituting with the proper procpid:

SELECT pg_cancel_backend(procpid);