How would I make a ondeath script that bans the player?

It's pretty simple to ban a player. And even simpler to run a function when they die.

Here are 2 examples.

Example #1: Server Ban on Death

local banList = {}

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        local humanoid = character:WaitForChild("Humanoid")

        humanoid.Died:Connect(function()
            table.insert(banList, player.Name)
            player:Kick()
        end)
    end)

    for _,v in next, banList do
        if player.Name == v then
            player:Kick("Banned from server.")
        end
    end
end)

Example #2: Permanent Ban on Death

local dataStoreService = game:GetService("DataStoreService")
local banData = dataStoreService:GetDataStore("BanData")

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        local humanoid = character:WaitForChild("Humanoid")

        humanoid.Died:Connect(function()
            banData:SetAsync(player.UserId, true)
            player:Kick()
        end)
    end)

    if banData:GetAsync(player.UserId) == true then
        player:Kick("Banned from game.")
    end
end)

So in both examples it works pretty much the same, a player enters the game then the script waits for the character to be added. After that it waits for the humanoid in the character and when the script detects that the humanoid has died it will run the code. In Example #1, it just adds their name to a table then searches for it when they enter the game again. If found, it kicks them. But in example #2, it changes a data store value to TRUE and when it detects a new player in the game it checks if that value is true and if it is, it kicks them aswell.

For more on Data Store click here.