Usage of a verb, Prevent

I would like to know when I can use prevent.

When I searched for the verb in dictionaries, it says

to stop something from happening, or stop someone from doing something Longman

So I thought I could use prevent even when the action they want to stop is currently happening. But my friend said I cannot use it when it is happening. She said rather, prevent only can be used before the action happens.

Can I use prevent after the action happens? For example, Bloomberg says:

As part of its continuing campaign to prevent China from stealing American intellectual property, President Donald Trump's administration is considering restrictions on the number of Chinese citizens enrolled at U.S. colleges and universities. Targeting foreign students will undermine U.S. competitiveness, not enhance it.Bloomberg

In this situation, does the US government think China is stealing their intellectual property? Or does the U.S. just want to prevent it before China steals it? Please help me. Thank you in advance.