Sure they can, they would just stop trading altogether...which has been done in the past...
Let a semi-orderly melt down happen over a period of days and not all at one time...
We should all know by now the Fed and UST will not allow a 40-50% selloff to happen in a single day or two, it can’t stop the sell off over time but it will stop or slow it down if for no other reason than to allow certain people to get out of the market in a orderly manner trying minimize loses.....
Go back to 1987 and that massive sell off, circuit breakers were put in place to stop trading for short periods of time to allow traders to take stock of what’s happening...
A couple of major difference now verus then is the use of high speed electronic trading that did not exist back then, the sell off will probably happen in the blink of an eye and a massive sell off will create panic...
The Fed will stop in and stop trading but a major sell off will still happen, if they open the next day it will happen again, etc....
The end result will be the same but it will just take a few more days....