How do I stop NSURLSessionD from starting every time I restart my Mac?
To permanently unload an agent or a daemon simply add a -w.

Example:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.nsurlstoraged.plist


However, the long term solution would be to identify what application is requesting downloads and controlling it. As an analogy, you can shut down a road temporarily to stop traffic, but long term, you want to control access to the road itself and not stop all traffic because some users are abusing or overloading the roadway.