You have three choices, IMO. Enter into the proposed agreement with your attorney, negotiate a different one, or institute fee arbitration proceedings. Since this appears to be a simple fee dispute which may result in litigation, while you may certainly file a complaint with the State Bar, I strongly doubt the Bar will take any action.
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If he will refund your money in exchange for taking down the negative postings, that would seem to be the best deal for you. Anything else would result in more litigation than it is worth. And what attorney would take your case when you are posting negative comments about your old attorney? He most likely would not be successful in taking action against you for negative posts if what you are saying is true. But you would have to spend more money fighting it. Just take the deal.
The agreement proposed by the attorney is absolutely inappropriate and against public policy / free speech and has nothing to do with returning unearned fees. The attorney is mandated to inform and offer you fee arbitration. You may also report the attorney to the State Bar.