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Honestly answer any questions, but avoid blaming. But, deliver the message clearly. It sounds like you think this person will have a hard time leaving you alone. You will need to be as specific in communicating that the business relationship is over at x date.
I have a few clients like this, however they pay their bills on time and never question the amounts I charge them. Sometimes you have to bite your tonigue and do what the client wants even when you know what they are requesting is not the best way to go.
I've only had to do that twice, but both times it was dreadfully necessary.
In one of those cases, my contract specified 50%payment upon completion. Well that job never did get completed, though I got it 90% of the way there. I let go and accepted the loss. Ya do what ya have ta do.