The only thing we can do as users at the moment is downvote them as low quality. But it’s nearly impossible to keep up with 5 to 6 high-rep downvotes required for each of these posts to give them the Low Quality stamp. Downvotes made by moderators (currently niche owners like yourself) do have a higher impact. I think using that voting power might be more effective than straight up removing the post from your niche as that would require the other 2 niche owners to do the same.
They all appear to be self-taken mobile pictures so they don’t violate the AUP. And if their rep gets low enough by downvotes they can just create a new account. On the other hand however, the AUP explicitly says that spam is forbidden. But who decides what is spam and what isn’t? That varies from person to person.
I hope that elected moderators combined with better spam prevention will eventually make this process easier.