Tax issues

I think I must be missing something very basic here, since no one seems to have the same question. I am helping a friend set up different tax scenarios for her German translation business. Within Germany she charges VAT; business clients outside Germany have to pay the VAT themselves (reverse charge) so she doesn't add it to her invoices except as a sentence reminding them that it is their responsibility to pay it. I can't figure out any way to just leave taxes off, it seems I have to choose a tax of some kind. Even if I call it "reverse charge" it seems incorrect to have a value of "0" because that suggests there is no charge. And I don't want to globally set the system up for no taxes because most often she does charge VAT. Is there a way to turn off the tax options for an individual invoice? Thank you!