DataStore ban system using keys?

So if I understand correctly, you simply want to store a table of bans into a datastore containing information about why they were banned, who they were banned by, date they were banned etc.

I would use something along the lines of:

local Datastore = game:GetService("DataStoreService"):GetDataStore("PlayerInfo")
local Banlist = {} --A table which we will use to store our data
local Key = "Banlist" --A generic key which we will store all of our data under

game.Players.PlayerAdded:connect(function(Player) --Will fire when player joins
	
	Banlist = Datastore:GetAsync(Key) --Will update the list every time someone joins

	for _,Bans in pairs(Banlist) do --Loop through the banlist
		if (Bans['userId'] == Player.userId) then --If they are already banned then...
			Player:Kick("You are banned from the server") --Kick the player
			return --Stop the function from continuing
		end
	end

	local moderation = Instance.new("Folder", Player) --You know this part...
	moderation.Name = "Moderation"
	local baninfo = Instance.new("BoolValue", moderation) --Are they banned?
	baninfo.Name = "Banned"
	local DOB = Instance.new("StringValue", moderation) --When were they banned?
	DOB.Name = "DateOfBan"
	local BB = Instance.new("StringValue", moderation) --Who banned them?
	BB.Name = "BannedBy"
	local UI = Instance.new("StringValue", moderation) --Guy who banned them's userId
	UI.Name = "UserId"
	local RS = Instance.new("StringValue", moderation) --Reason for ban?
	RS.Name = "Reason"

end)

game.Players.PlayerRemoving:connect(function(Player) --Will fire when the player leaves

	if (Player.Moderation.Banned.Value) then --If they were banned

		local Moderation = Player.Moderation

		Datastore:UpdateAsync(Key, function(Bans) --Update the datastore ("Bans" is the list already stored)
	
			table.insert(Bans, {
				['userId'] = Player.userId;
				['DateOfBan'] = Moderation.DateOfBan.Value;
				['BannedBy'] = Moderation.BannedBy.Value;
				['BannerUserId'] = Moderation.UserId.Value;
				['Reason'] = Moderation.Reason.Value;
			})  --Add the ban to the banlist

			return Bans --Update the list
	
		end)

	end

end)

There are better ways of doing this, but I don't have much time. If you have questions, please drop a PM or comment.

If you want help with the datastores, or techniques which I have used here, check my answer on this question.

This script only handles the saving of the data, if you want to ban/unban them then you will need to write separate code