Poll of the Week: Directive squared

You are the captain of a Starfleet in unexplored territory. Your crew discover a civilization, approximately equivalent to middle ages Earth, but with surprisingly advanced mining equipment. Since they are not warp capable, the Prime Directive prevents you from interacting with them, but you wait in orbit to study their culture.
In a few weeks of study, you realize they have a really active polytheistic religion, and their gods seem to take a very present role. You also see how they offer sacrifices to their gods in the form of a very particular mineral uncommon in this sector. Soon enough, your suspicions are confirmed as an alien ship comes to take the mineral and interact with the species. The new species are technologically advanced, but definitely not gods.
Now, you know the Prime Directive prevents you from interacting with them, but the alien species are not subject to such a code. Leaving them be would be leaving an innocent species live in slavery, but stopping the whole exchange would be paramount of playing gods yourselves.
Starfleet Captains have found themselves in similar situations in the past. Now tell us, what would you do?
This is a new edition of our category Morals of Trek, where you are in the shoes of a Starfleet Captain facing a dilemma any of our favourite characters could have faced in Star Trek. If your crew has faced any such dilemmas and you want to see it featured in a Poll of the Week, let us know!