Customer returning no longer needed items

Hi.
I am looking for advice.
I have a customer who has bought 4 cake decorations, each one a separate transaction, but has now submitted a claim stating “No Longer Needed”
I am FBM as so these are auto authorised and all 4 have separate tracking references to return so I am guessing I have 4 retruns to pay for.
How do i stop this person from buying from me again?
Can I complain to Amazon about them?
I know I will get these every so often and its par for the course but 4 from one customer? Hopefully there is something I or Amazon can do?
Any advice would be appreciated