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.
You're right; however, trade sanctions by the US and other industrialized nations weren't implemented until sometime in the mid-1980's, that is what I meant by ending Apartheid within "a few years." By the same token, Reagan's arms build-up was basically a form of economic sanction against the Soviets and that also worked within a decade. If you examine the past century, you will see that no nation has ever prospered economically if the United States was actively working to apply economic pressure.