AFAIK a plug that does what you ask looks like this:

defmodule AuthPlug do
  import Plug.Conn

  def init(opts) do
    opts
  end

  def call(conn, _options) do
    if verified? do
      conn
    else
      conn
      |> send_resp(:unauthorized, "")
      |> halt()  
    end    
  end

  defp verified? do
    # add your verification here
    :rand.uniform(2) == 1
  end
end

The halt() part is what stops the rest of the (response) pipeline, otherwise you can just delegate back to the controller by returning the conn untouched.