The general consensus I’ve seen online is that there aren’t many practical ways to stop people from pirating your game. However, it’s actually possible to turn piracy to your advantage, especially if you release updates for your game regularly. I’ve read that the people who run/contribute to these type of sites are often more focused on finding new titles to pirate than on keeping every single game in their libraries up to date. If you keep up a steady stream of new content, then the people who download your game from the pirate site are more likely to get an outdated version, and if they like your game, they’re more likely to become customers of yours.
It’s also worth mentioning that there are a couple types of players who pirate games: some pirate specifically because they feel entitled to free media - these are the people who see a game on Steam and go straight to pirate sites to check for copies, and if they don’t find it there, they just move on to other games. You’re very unlikely to ever see any money from them, whether or not pirated versions of your game exist. Other players pirate games because they want to try before they buy, and if they like the pirated version of a game, they’ll go to an official source later to purchase the real thing.
Some devs have reported increases in their monthly sales figures after their games were pirated, since it created more exposure for their games and let them win over customers who were on the fence about purchasing. I’ve read that sometimes devs will even go ahead and release their own games on pirate sites to get ahead of actual pirates. Of course, I’d recommend researching that practice before trying something similar yourself, since it could conceivably backfire if you’re careless.
But yeah, sorry to hear that this happened, and I wish you luck with figuring out how to take these guys down. But if that doesn’t work out, hopefully it’s some comfort knowing that you might still be able to turn this experience into a net positive for your game.