1) The Feds have the guns. That specifically includes the general threat of guns-out federal agents storming the Microsoft campus for trumped up reasons to make a raw show of power. Microsoft's stock would plunge.
2) They have the IRS, Homeland Security, DOJ, FCC, FTC, SEC, and every other agency they could possibly need to break Microsoft's leadership and its board of directors. It would be done within a month and would be tragically easy.
Just the smallest potential of this type of scenario would have MSFT shareholders shitting their collective pants.
The US Government directly lords over all interstate commerce, in dozens of ways. They could, within the span of a few days, rule that Microsoft was impeding interstate commerce, posing a danger to national security, and they could order federal agents to take control of Microsoft; that wouldn't occur, it would merely be hinted at, at which point most of the board would immediately resign as would the CEO; the US Government would then watch as a new temporary board and CEO is installed. The US Government is able to force ports open - if need be - on the same basis they would use to force Microsoft 'open.' They could do it with either a soft touch, or be as forceful as necessary.