The Trump administration intensified its crackdown on foreign students coming to American universities, as the State Department instructed American embassies not to process new visa applications. The government intends to strengthen its vetting process. That includes trawling students’ social media for anything inflammatory, such as support for Hamas. Marco Rubio, the secretary of state, said the administration would also “aggressively” rescind the visas of Chinese students, who normally eschew campus activism and make up a quarter of international students.