You've accepted another offer, you've given your leave and you're moving on. There's nothing they can do about it. You don't need to tell them anything. If they ask where you're going you can simply say that you had 'another opportunity' and you've made up your mind. If they try to make offers to get you to say, you can simply thank them and decline to stay. Every single time.
I know you might think it sounds silly saying the same thing over and over again, but the reality is they're doing and asking you the same thing over and over again, so it's a perfectly valid and professional response. And you're right, it's better not to decline the meetings, just agree to them and repeatedly let them know your position. As @Kilisi saidKilisi said, if they want to waste time and energy trying to get you to stay instead of accepting it and looking for someone else, that's NOT your problem. Don't let them pressure you, this is your decision and you have autonomy. You can't control what they do, only your responses to their actions.