I don't think this is America's problem to solve. I do think that America can impose trade sanctions against the countries that knowingly allow this to happen. We did this in South Africa and Apartheid was over within a few years (even though the system that replaced it isn't any better, it's just different).
I agree, trade sanctions are an option. However apartheid was around (on the books at least) for more than just "a few years." It became official policy in 1948 and wasn't removed until 1991.
There is no telling how long this horrid injustice will continue in Cambodia, even with a US embargo in place.