To the best of my knowledge, reading over docs for outputs.conf, I don't think what you're looking for is possible. You could try changing the autoLBFrequency to a high value, but that just means that the forwarder would develop a long-lasting affinity for whatever indexer it's talking to. In a failover event, it would hang on to its connection to the secondary for a long time.
Let's dig deeper, however. I'm curious to know why you'd want data to go only to a primary indexer, and not both. You'll get the redundancy you request, as well as increased search performance if you utilize both indexers together. Is there some reason why you wouldn't use both? Is it simply a matter of bandwidth? If so, note that you can enable compression on both sides of the connection (forwarder and indexer) to decrease overall bandwidth usage.