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How can I stop a bar stool from rising my itself when no one is on it?

I bought four bar stools about 2 years ago, and recently one of them has been up at maximum height every morning. We keep them at minimum height, as that is best for our counter. How can I stop this stool lifting by itself? I happens every time there is no one sitting on it for longer than half an hour or so.
Maire, April 2013