Online stock trading platform Robinhood made the controversial decision to ban its users from buying certain stocks last month after the company that clears stock transactions imposed a $2.2 billion special charge on Robinhood. After the ban, the clearinghouse lifted the charge. Robinhood was popular among the small investors who organized online to buy the stock of GameStop, a struggling computer game retailer. The run was partly motivated by a belief that hedge funds that borrowed and short-sold the stock would be forced to buy it back, thus increasing its price. As a mass of investors bought in, the price indeed increased from less than $5 per share in August to $30 in mid-January and then nearly $500 per share at one point on Jan. 28. But in the morning of Jan. 28, Robinhood blocked its users from buying the stock, which has since dropped to some $40. The company …