I would right his forwarding address on the mail or write "Not at this Address.". That would get him moving. While I fully agree that your kids relationship with him is between them, I don't believe he should come and go as he pleases. I would change the door code and at least set times when he can be there. If he balks it is easy to say, you wanted this and this is the way it will be when we are divorced.
I will say since I am divorced, I do accomadate him changing a day now and then. Really this is for the kids as long as it doesn't mess up what I already have going on. He has tried to mess up my holiday time and I do put my foot down. Don't let him dictate your life to you.
kat
Me-53(and learning!) S24, S21, D18, D17 Just keep swimming, Just keep swimming. Dory