Personally, what I find valuable about art is that it's communication from another human, so it matters to me. I'd prefer the human-written story even if the machine-written one is technically better.
Sort of like how I prefer handcrafted pottery over machine-made pottery, or handcrafted furniture over machine-made furniture even if the machine ones are technically superior.
Require a non-trivial but affordable deposit to review the submission. Refundable if an actual effort was made whether or not it is accepted. If I just spent the effort to actually write a story, I wouldn’t mind depositing $20 to get someone to review it, with full expectation that I get the money back.
Or as they say “While rejecting and banning these submissions has been simple, it’s growing at a rate that will necessitate changes.”