curl -u admin:admin -i -H 'X-Requested-By: ambari' -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.