I think that the only way to reach these grave robbers would be to sentence them to having the graves of their own ancestors dug up and sold off as artifacts. Then maybe they'll get it.
I agree that the artifacts shouldn't just be squirreled away by museums unless some of the museum displays could be enhanced.
And I think that Native peoples could have these relics returned to them so that they could decide on their disposition. The problem would be that it's not clear who the descendents are of some of these peoples. For example, while many artifacts are on Navajo land, clearly these aren't the Navajo's ancestors.