Make it clear that there will no discussion. Just SAY it. "I'm going, it's final, I've made up my mind so I'd appreciate that there's no discussion".

If you're certain to leave, do it and don't fret about it. And never ever care what they have to say about it. After all, friendship and work relationships mean nothing when it matters. They will gleefully fire you without hesitation if needed, with or without being very good friends with you.

Always remember that.