If the user is not connected:
To make a character choose again the next time they log in, change that character's Status to Pending in the relevant file at BepInEx\config\DiscoverySync\worlds\<world id>.json. Their login will then trigger Munin to revisit them and they will be considered opted out until they answer again.
If the user is connected:
Stop the server and change the value, forcing them to log back in and trigger the decision.
Deleting the entry also works, but their discoveries then show no discoverer.