Surely the only real answer here is seperate progression for offline and online players? Those who only play offline can go do that against bots and unlock whatever but those unlocks won't appear when they go online. And then just don't reward any XP when you kill a bot in online servers.
They can develop as many "smart" as they want but I honestly think that there's almost no solution to stop the xp servers from existing
Edit: the only solution i can come up with is to only enable progression on paid/rented official servers which also somewhat address the question of community wanting to rent their own server and even that won't fully kill the xp servers. Hell, it'll probably trigger even more people
