You use the broken record method. 

Whatever they say, your answer is “I have given my notice, and my last day of work is the 6th of June”. If they ask why you are leaving you say “I have given my notice, and my last day of work is the 6th of June”. If they ask what it takes to make you stay you say “I have given my notice, and my last day of work is the 6th of June”. Whatever argument you say “I have given my notice, and my last day of work is the 6th of June”. 

Manipulators are good at making arguments that are hard to refute for someone who isn’t an experienced manipulator themselves. And some people think because they can’t refute the argument they have to do what the manipulator asks. The broken record method makes it clear to the manipulator. On the other hand, you can just say to the manipulator “your arguments are much better than mine. The answer is unchanged. I’m leaving.” Or say “No”.  “No” is a complete answer.