Ask HN: How to best handle accusation of plagiarism/copyright?

This bothered me a few years ago, but I did not have the right forum to ask, but now that I caught the startup/hacking bug again, it's fresh in my mind.

A few years ago, I wrote a simple Facebook game based on a public domain game, let's call it Tic-Tac-Toe. (it wasn't, but it was in PD) It was created in fun over a weekend, at some friends' insistence, with minimal UI. (Facebook colouring and style) It's not super popular, but grew to a few thousand, without any of the spammy techniques. At some point I was contacted by another developer of the same public domain game, saying I ripped off the concept, UI, and all sorts of things. I checked it out and his game has over a million users.

I was distraught and basically gave up on the app, though it's still up now, without much change, though a few heated words were exchanged.

What is the best course of action in a situation like this?

tl;dr Another dev accused me of ripping off his app, I got upset and gave up.