If the syslog processor is configured with indexAndForward=true, all events sent to the processor (i.e _SYSLOG_ROUTING=routeArcSight) will necessarily be indexed. I'm not sure how you would conditionally disable that. Setting queue=nullQueue would just prevent the event from being sent to the syslog processor in the first place.
You might try to configure the syslog processor with indexAndForward=false and then for those events that you want indexed also add queue=indexQueue. But I think you would bump up against the same issue - if you use transforms to send an event to an output processor, you can't also send it to another queue.