The White House should simply promulgate strict rules: "The management reserves the right to limit quantities in the interest of fairness. One to a customer." Hand out printed notices to each journalist as they enter, and note that the rule will be strictly enforced.
One question, no rebuttal. Journalist asks one question -- President (or Sanders) answers one question. At the conclusion of the ONE question, the microphone is taken away from the journalist. No follow-up on the answer. At the conclusion of the ONE answer, the microphone goes to the next journalist. Anyone who refuses to play by the rules is removed for the duration of that presser.
Make it a lottery -- each journo draws a number out of a fish bowl as they enter -- that determines the order in which they get the microphone. Just like the deli counter at the supermarket.