>it's good old fashioned hard work: hire more bin men, and put up more information posters

I don't think that's the primary problem either. "Bin men" must be paid and they are going to be too expensive, and "information posters" irrelevant, until the populations surrounding these rivers can reach a standard of living that gives them sufficient resources to pay bin men, and to care about what materials they need to put into which brightly colored bin.

In other words: fix GDP of these countries by improving global economic growth, and you will cut off the flow of plastic at its source.