Then the law needs to change. It's unconscionable for this to be happening. You make the buyers of that cobalt start caring, you'll make the producers of that cobalt start caring.
Or the producers of that cobalt will just find someone else in Shenzhen to sell to. This problem should be tackled, but it is worth thinking about likely unintended consequences of whatever power structures you set up to tackle it. I hear the Belgians have some experience ending slavery in parts of Africa.
This trade is difficult to break in the current environment. We'd need to get tough on people who we have generally not wanted to get tough with because we want to keep them friendly for a lot of reasons. Not least of which is growing Chinese (and European) influence in the nations of coastal Africa. We know very well that some of the people we need are less than savory. Some of the trade we engage in is less than honest. But it's not just the coltan trade that's influencing our behavior here. There are a lot of different and competing strategic considerations at stake.
My own opinion? This is probably going to blow up in our face in the future, and we'll spend the latter half of this century, (or maybe the first half of the next?), attempting to convince a rapidly developing sub-saharan Africa that we're deeply sorry for the past but you can trust us going forward.