THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
curl -uadmin:admin -i -X PUT -d '{"Requests":{"request_status":"ABORTED","abort_reason":"Aborted by user"}}' http://localhost:8080/api/v1/clusters/$CLUSTER_NAME/requests/$REQUEST_ID
Note that this currently does not work for aborting logical requests created from Blueprint API calls (as of Ambari 2.1.2).