GameStop and the Means of Prediction Requiring cash on hand or 100% margin for a volatile stock is entirely reasonable and banks and brokers can freely set these margin levels as they see fit. The clearing broker and clearing members are guaranteeing all these trades will settle vis-a-vis all the other participants of the clearing system. That allows everybody else to trade without knowing their counterparty and whatever settlement risk might be involved with that. From a risk management perspective it's necessary to allow a lot and that includes telling traders to fuck off with orders unless they post additional margin.
Banning trading in a specific stock or derivative is possible too but with a reasonable notification period - which can be very short in volatile markets. For instance, I once bought KIOR stock and that tanked into penny stocks and the costs for the bank to pay the deposit fees were no longer in line with the value of the underlying stock so they told me to get rid of it or they'd close out the position for me on a specific future date.
Problem with Robin Hood is that margin lending is embedded in its use case as far as I can tell and if hordes of dummies lend margin to buy a volatile and shitty stock then quite obviously banning them to buy, in order to protect the recovery rate on those loans, is an obvious risk measure.
So quite frankly, I think the outrage is totally misplaced.