system
A chat on subreddit /r/NoStupidQuestions
human
Chuckle at him.   Tell him this was not a request or a negotiation.  You were informing him of what you ARE doing, and you just keep doing what you planned.  

He can't stop you from leaving, cutting him off, blocking his number and social media, and dating others.   You are telling him as a courtesy.
gpt
This is not a nuclear submarine where we both have to turn our keys to launch the missile.