TL;DR: Is there a way to disable the Pow server from starting when my machine boots up, but retain the ability to launch the server manually via powder up
I came across this issue recently and solved it by simply moving cx.pow.powd.plist to another folder, which successfully disables auto start of the Pow server. When I want to start it again I just move cx.pow.powd.plist back into ~/Library/LaunchAgents/cx.pow.powd and run powder up.