It's a question of the base ethics of providing a platform; not about the legalese and the loopholes you can refer to and weasel out of it. If you provide a platform on which there's a clear and recurring pattern of abuse, and if you don't do anything about it, it begs the question, "Why did you do it to begin with?" Was it your intention to just put up something and let everyone run wild? While I have sympathy for the issues you face in dealing with situations like this, the approach doesn't see more came across to me as responsible, mature or proactive.

Given the amount of shady business going on on the internet, if you aren't willing to deal with keeping it straight and protecting those who really invest their time and effort into it, perhaps you shouldn't do it to begin with. That'd save you from all the trouble of dealing with complaints, whether from original authors or scammers and plagiarists who were shut down. I have grown to be disgusted by plagiarism over the years, and I've seen it on too many markets and in too many contexts, as well as been in the suffering end too many times, to have any sympathy whatsoever to anything that actively supports profiteering by copying the work of hard-working people, often indie developers who may actually *depend* on this for their livelihood.