Look to your written contract for the way to terminate the contract. If it doesn't say, then send him a letter, certified, return receipt requested, stating that you are terminating the contract based on his failure to communicate and that you demand a refund of your entire deposit within 30 days. Do not pay him anything else, and immediately hire another planner, so you don't foul up your wedding plans. Document everything with the bad planner. If he doesn't return your deposit in the thirty days, file a small claims action to require him to return your deposit. Since it is less than six months from your March wedding, it is possible that the small claims judge won't find in your favor, but there is good chance he/she will on these facts. Just make sure you have good documentation that the guy wasn't communicating with you for all these months, and that you can convincingly show that his conduct was unreasonable on the terms of the contract. I think the penalty of 50% of the entire balance would be deemed punitive and unenforceable. If the amount of the entire contract exceeds $10,000, you may want to hire a lawyer to look carefully at your contract and advise you.
You may not be able to avoid it without going to court.
Send him a letter telling him that you no longer want his services as a result of his failure to communicate with you. Demand your money back.
If he gives it back, great.
If he does not, then you will have to sue in small claims court.