Easy answer, stall them.
"An emissary from the Emperor is on his way".
Or, "Light speed communications round trip time is 3 days. I am no more empowered to make decisions that are enforceable on the Emperor that XYZ ship captain could make treaties enforceable on the Prophet."
You are almost certain to lose the planet in any case. All you can do is make sure the Axon fleet does not get away, not without revealing the warp point they came through.
Alternatively, I would threaten. And I would not threaten genocide. No, I would couch the threat as so:
"We will invade all Axon worlds. True, you can seek matyrdom and destroy your own worlds. But all surviving Axons will be taught from childhood in ASR or Empire schools. They will be taught history by us. So any Axons who survive will be one who are raised to hate the Prophet and reject his faith. We have the wealth and the means to corrupt all who you would call weak in the faith."
"Now you know better than anyone that the faith of some Axons are weak. And yet, only those Axons who are weak in faith will survive. And those Axons who are weak in faith will prosper, we will assure that. And it is possible that the Axons will go extinct. You have my word that the breath of the last Axon will be spent cursing the Prophet and all of his works."
"The fate of the world you threaten, and the fate of the faith of the Prophet are in your hands, and their fates are the same. Destroy the planet, and the faith in the Prophet will be destroyed. Show mercy, and the very weakness you scorned will allow it to survive, but not to thrive."