strip_params.ex

defmodule MyPlugs do                                                                                           |
  def strip_params(conn, param_key) when is_binary(param_key) do                                                                              |
    param = Map.get(conn.params, param_key) |> strip_param                                                                                    |
    params = Map.put(conn.params, param_key, param)                                                                                           |
    %{conn | params: params}                                                                                                                  |
  end                                                                                                                                         |
                                                                                                                                              |
  defp strip_param(%{__struct__: mod} = struct) when is_atom(mod) do                                                                          |
    struct                                                                                                                                    |
  end                                                                                                                                         |
  defp strip_param(%{}=param) do                                                                                                              |
    Enum.reduce(param, %{}, fn({k, v}, acc) ->                                                                                                |
      Map.put(acc, k, strip_param(v))                                                                                                         |
    end)                                                                                                                                      |
  end                                                                                                                                         |
  defp strip_param(param) when is_list(param) do                                                                                              |
    Enum.map(param, &strip_param/1)                                                                                                           |
  end                                                                                                                                         |
  defp strip_param(param) when is_binary(param), do: String.strip(param)                                                                      |
  defp strip_param(param), do: param                                                                                                          |
end

Use in controller as:
plug MyPlugs.strip_params "field"