Disable tax calculation on an order in Spree Commerce

tax_rate_decorator.rb

Spree::TaxRate.class_eval do
  # UPGRADE_CHECK https://github.com/spree/spree/blob/1-3-stable/core/app/models/spree/tax_rate.rb#L33

  def self.adjust(order)
    order.clear_adjustments!

    # TODO you'll have to customize this based on your situation
    if order.taxable_flag?
      return 
    end

    self.match(order).each do |rate|
      rate.adjust(order)
    end
  end

end