To come to this conclusion, I assume that you know who did build the websites this seller uses in their portfolio and that you have contacted each design company to ask if they ever outsource work or are connected to a seller on Fiverr in any way?
I mean otherwise, all you are really doing is running around shouting "Fiverr seller who says he offers web design services, has websites in his portfolio which look quite good."
If you know anything about web design, you will also know that it is possible to resell different themes and plugins which often do still show the original creators branding. I don’t sell web design but I have a license which allows me to sell several themes created by a group called OceanWP.
I would advise you think carefully before you let what seems like an unfounded grudge/manic moment of boredom, spur you to attempt to ruin a random strangers career. The universe is watching and occasionally, karma gets hungry.