Sounds as if your worries might not be unfounded.
However, you’re caught between Scylla and Charybdis.
You can send them a cancellation request via the Resolution Center, giving a cancellation reason from the choices that you can live with. If they accept the request, they’ll be refunded by Fiverr (attention, minus the service fee, which might make them upset at, guess, you, again). They can’t leave a public review then.
Or you can take the stance that it’s their problem if they order something they don’t understand and that you deserve to be paid for your time (very reasonable stance).
In this case however, they can leave a bad review, and, from your observations, probably will.
Then, of course, theres always a 3rd monster hiding behind the rocks, they might go to support, complain and get them to cancel and refund.
As the problem with all scenarios ending in a cancellation is that it will affect your order completion rate, I’d say get proactive and contact support first, proving to them that you delivered what your gig promises and that it’s in no way your fault and see what they say, at the least, if it will come to a cancellation after all, in the end, you might get a support person that tells you in case the drop in your rating caused by that cancellation would lead to demotion won’t lead to level loss if you contact them about it before next evaluation date (if you’re level 0, you can ignore this bit).
They might tell you to work it out with the buyer (probably not possible from what you say), they might even have your back by telling you to not accept a cancellation from the buyer, but that will still leave you with a bad rating, probably.
Most people will, with reason, say don’t cancel, especially if you think it’s a scheme, but you’re probably aware that a bad rating, depending on how many you have, how many regular customers you have, etc., can be a bad hit for your account. Tough decision to make.
Could you possibly get along with the customer somehow, after all? Find out if they honestly were confused about what they’d get from your gig and what they actually expected from your gig and do that in case you even can?
Personally, from what you’re saying, it sounds like a scheme which I’d not want to support, so I’d tell them I delivered exactly what the gig said and spent my time on it and that I’ll contact support about the issue and see what the buyer says then.