Paper has a built-in Anti-Xray feature that hides ores and other valuable blocks behind fake blocks, making x-ray texture packs and cheats much less useful. It only works on Paper and forks of Paper. See Server Jars if you need to switch.

Steps

1. Open the Files tab and go to the config folder.
2. Open paper-world-defaults.yml.
3. Find the anti-xray section under anticheat and set enabled to true:

anticheat:
  anti-xray:
    enabled: true
    engine-mode: 1

4. Choose an engine-mode:
   • 1: hidden blocks are swapped for the normal block of that dimension (stone, deepslate, netherrack or end stone). Lighter on players’ computers.
   • 2: hidden blocks are mixed with randomly placed fake ores. Harder to cheat with, but can lower FPS on weaker devices.
5. Save the file and restart the server.

Anti-Xray is not perfect. For serious servers, pair it with an anti-cheat plugin.