As a last resort you might reflash the phone yourself using
Odin. You would need to download the correct firmware from
Sammobile or
Youmobile then flash the phone as shown in the Odin tutorial I linked. It's not a difficult thing to do, but stipulations are...
1) You absolutely must use the correct firmware for your phone model.
2) The firmware must be for your carrier. Do not flash any other carrier's firmware.
3) The firmware must be for your country. Same here... do not flash any other country's firmware.
If you flash a wrong or corrupted firmware, wrong carrier, wrong country, you can brick the phone... more than likely soft-brick, but even that's no fun. If things go well, then the phone may turn on without the messages, however, you'll still be left with the blacklist problem. The flip side is that perhaps even a successful flash won't get rid of the messages.
Flashing firmware is not without risk and if you're a computer guy, then you'll understand that it compares to flashing bios. Personally, I've never had a firmware flash fail, (bios either, for that matter), but my luck is my own.
Up front, I'll tell you that recovering from a bad flash is sometimes very tough via a forum. Mostly due to the fact that the phone isn't in front of people you're asking help from, not to mention the error message can be rather grey as far as specifics are concerned. There are quite a few experienced people here on AC, but most reside on the XDA forum. That said, if you attempt the flash and it fails, I recommend you join and post to the XDA Note 4 forum... I think Sprint has it's own forum branch.
Finally... the last ditch effort is rooting the phone. What that might gain you, I can't speculate. Rooting is something I'm not familiar with on the Note 4 as the last phones I rooted were my HTC's, and until just recently, Verizon Note 4's were not rootable, so I never even looked that deep into it. Again, XDA is the place to visit for any/all information about rooting. Rooting is also not without risks. Personally, I'll never attempt a root on my Note 4 simply because the risk is too high... for me.
I apologize for my long-winded posts. Just trying to anticipate any questions you might have. And no, you more than likely can't revert your phone to Kitkat.