Everyone will experience this at some point. Sometimes a customer just doesn’t like what you’ve done other times they’re fishing for a freebie. You need to be a little bit psychic to work on Fiverr, to read past what the customer gives at the start.
You’re under no obligation to refund their order if you’ve done what they’ve asked for. If they say they don’t like it, ask them to expand on what they don’t like about it so you can address the issue for them. I glanced at your gig and see you don’t offer revisions, that puts you in a slightly more difficult situation as it says to the customer you won’t make any changes, but that is your prerogative.
I’ve had customers pull the “sorry, it’s just not what I wanted” but I absolutely know I’ve done exactly what they’ve asked for or exceeded what they asked for and thus have said that it’s unfortunate they don’t like it but unfortunately I don’t offer refunds after the work has been done. For me I am somewhat flexible on changes so 9/10 times I’ll sort the issue out with them and everyone is happy.
You just need to decide whether an order is worth a potential negative review and a hit to your completion rate or not. If you’re confident what you’ve done meets their order brief and you want the money then refuse if not cancel and move on.