Like getting logs in PSO2 Meseta programming, a lot of time you read , but when you do... you find out just how easy could it has been solved xD You could try contacting their support staff right possibly. If not ingame then there's more then likely an out of sport option to pursue.
Unfortunately, I find myself at a loss for what to do in the specific case you point out due to SEGA not providing any tools when you understand the offending party, but you do not have a PSO2 participant ID to tag them with. It's like, understanding their faces, understanding where they are. . .but not their names which the player report formats require to report any malicious behavior. Yes, I assessed the ingame and out-of-game official tools they had in plain sight, just to be certain.
Then again I'm pretty inexperienced myself therefore I didn't check everything there was to explore as far as electronic options go (more for me to look up I suppose) If anything. . If you manage to take down one of these meseta sites, another will pop up in their place and continue running as if nothing occurred. Domains are cheap and it doesn't cost much to rent out a machine to host the website itself.
Even if you might, or sega could, take down one of these sites, they'd only make another. They've an infinite supply of buy Phantasy Star Online 2 Meseta mails and bogus credentials to make new characters, with proxy or ip hiding applications so no action can be taken on the people behind them. They're not scumbags, they're developers searching to get simple money using virtual currency, bot programs, and manipulation of files.