Sorry, there is no workaround at present.
We have a fix slated for 5.0.5 which will log messages to splunkd.log. It will log message for only first 10 forwarder instances not using ACK.
Sorry - that doesn't answer the question. It's like the old joke, Patient: "Doctor, it hurts when I do that." Doctor: "Don't do that."
I've submitted an RFE to be able to disable the warnings.