Each world's shared discoveries are saved on the server in BepInEx\config\DiscoverySync\worlds\<world id>.json. Stop the server, delete this file, and restart. The file also holds every character's choice and who discovered each item, so those are reset too, and every character meets Munin again on their next login.

Note - what's done is done

Individual discoveries that characters have made before this point cannot be reversed. Valheim does not have a mechanism to remove or "forget" discoveries, so no effort is made to revoke them, primarily to avoid save file corruption.