How do I remove hats onjoin?

If you want all the hats a player has to be destroyed everytime they respawn, then instead of using waitforchild or findfirstchild, you should use the PlayerAdded event and CharacterAdded event to search through the entire character for hats using the :IsA() method and then destroy the hat.

local players = game:GetService'Players' -- Get the players service

players.PlayerAdded:connect(function(p)
    p.CharacterAdded:connect(function(c) --This event will fire whenever the character is spawned
        wait(.8) -- Hats take a few second to load
        for i,v in next,c:GetChildren() do --Search through the entire character
            if v:IsA("Hat") then -- Check if v is a hat
                v:Destroy() --Destroy the hat
            end
        end
    end)
end)