If you want to completely disable raid leaderboard notifications, just unregister notifications provider:
Lua Code:
EVENT_MANAGER:UnregisterForEvent("Notifications", EVENT_RAID_SCORE_NOTIFICATION_ADDED)
EVENT_MANAGER:UnregisterForEvent("Notifications", EVENT_RAID_SCORE_NOTIFICATION_REMOVED)
Right now I'm trying to figure out how to disable notifications just for selected guilds.