I posted a project for original artwork for various prompts. Two of the freelancers I hired had the same ten prompts, and two of the images they submitted were the same as each other, which caught my attention and led me to do reverse image searches, which led to me finding the images on Shutterstock and Adobe Stock, along with some other ones they submitted. The first freelancer I had already paid 3 weeks ago, the other I haven't yet paid. I called Upwork and got a response that they aren't able to discuss plagiarized artwork with anyone but the rights holder of the art, which, I can understand, but I think misses the point of my concern.
It seems unlikely to me that easily verified plagiarized works would go unpunished, but I also might be naive - are freelancers generally able to submit stolen work as their own? Did I get a poor customer service rep? I know I shouldn't have accepted the work from the first freelancer without doing more verification, but the freelancer had a lot of completed jobs and a high success rate, and I'm fine eating what I paid, but I really hope that there's some way to prevent someone I can prove stole work from being able to con future clients.
re: "Freelancers submitting art from Shutterstock / Adobe Stock as their original work, what can I do?"
Stop working with any freelancer who does that.
Instead, work with freelancers who take pride in their work and who only provide original work that they themselves created.
re: "It seems unlikely to me that easily verified plagiarized works would go unpunished, but I also might be naive - are freelancers generally able to submit stolen work as their own?"
Only a dishonest, dishonorable freelancer would do something like that. The overwhelming majority of Upwork freelancers would never even think of doing something like that. But Upwork is the industry leader in remote freelance work, and among its millions of freelancers there are many who do as you describe.
Ultimately, you will want to find effective strategies for doing this on your own. It is not Upwork's intention that they be put in the middle of such situations.
Upwork wants you to use the powers you have as a client to hire and fire people, to continue working with the good ones, to leave appropriate, accurate reviews of the bad ones, etc. Upwork doesn't want you to call them or otherwise call upon their employees and customer support staff to help you manage your project.
Without knowing the ins and outs of the project, the first concern that I had was regarding the image licensing. In addition to the problem with lying about producing their own work, the images that they did use were released under some sort of license and you don't know exactly WHAT the license covered. If you wanted to resell your product or have it seen by a massive audience or viewed at an extreme scale, that may not have been covered by said license and you would have been held accountable if you chose the art and published it.
On the other hand, while the artwork that they used may not be their own, if they tweaked it enough to make it "uniquely" their own, then they could still legally call it "an original work" and free you of the licensing restraints.
There is, of course, also the possibility that (one of) the freelancers did create the artwork. Every bit of artwork on Shutterstock etc was created by someone, mostly by freelancers.
I am, of course, not saying that this is what happened, but it's possible. If I had work on Shutterstock, that work would also be in my portfolio!