There are probably a few ways the president could do it unilaterally, legally:

1) Raise tariffs to absurd levels (like, 10000% tax on anything from China).

2) Some sort of national defense justification. He could probably put all employees, companies, and executives on terrorism or customs watch lists and make life difficult for them.

3) Use existing sanctions on other nations as a way to forbid doing business with various entities in China.

There's probably also another way to do it that would, presumably, be illegal:

4) Unilaterally blockade the nation, since he's chief of the armed forces (this is an act of war).