1) Forbid any trading of stock futures. To me, reading about stock futures reaching circuit breakers would invoke panic and the need to sell as soon as the market opens. Let the market open at 9:30 EST, to whatever price the market feels is appropriate at that time. Do not give them a clue as to what it can be. Let the market decide for itself.
2) Forbid short selling. I feel most of the selling is done by long-term holders, but short selling does excacerbate the situation. I'm in favor of forbidding short selling forever, let alone in a crisis, as it's an artificial technicality of the market. You can't short sell houses, or your possessions, right? So why can we short sell stocks (if you want to hedge, go buy put options)? I sure as heck didn't give anyone the right to borrow my stocks and sell it. It's mine - I own it.