Help! Moomoo charged me $ because I bought with margin, how to stop interest?

Take it as a learning experience. My suggestion is to play with paper trading first.

When you buy stock with margin, you are borrowing money from Moomoo to pay for the stock. Of course you would be charged some borrowing cost and you can't stop it as long as you still owe them money.

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To stop paying margin interest, you either have to top up your account with cash as suggested by @1M45, or you sell the stock you bought with margin. For the latter case, if your sales proceeds are not able to cover your buy price and margin interests, your account will have negative balance, meaning you still owe Moomoo money, and you have to top up your account to return the balance to zero or positive.