Well since we're constantly digesting our own dead microfauna, I'd say that it's literally impossible to be fully vegan, so they might as well stop trying and spare us their obnoxious bullshit.
"Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals."
~ The Vegan Society, 1944