AIUI they can't do that under the terms of the deal. So unless the deal is well and truly done, they can't stop him from tweeting.

E: Given I can't find sources on this right now (gosh, Google really has gone to the shitter; I can't find the SEC filing for the final deal despite several tortured queries) I'll say, take this with a pinch of salt. If I find a source I'll post it.

E2: I finally found it: the same non-disparagement clause allows for Elon to tweet whatever he wants about the deal. Section 6.8 in https://www.sec.gov/Archives/edgar/data/1418091/000119312522...