You can block any user you want on ancestry.
My trees have always been private but I have always answered anyone who has contacted me. I have a stripped down tree at FTDNA but would prefer only a striped down tree on ancestry to my DNA matches only. This option is not available at this time so I will chose to keep our tree closed off. This account I am posting on is a shared family account. Im not the only one who uses it and not the only person who has DNA tested on it.
You may not have the adoptees tested in your family but we have the adoptees tested and also have matches at FTDNA to people who were also adopted who have no family tree. People can't post what they dont have and adoptees, sperm donor/egg donor children will be reluctant to tell you about their situation for fear of rejection.
Also at FTDNA our family has matches to actual American Indian Tribal members and we also have documentation of AI in our family though not enrolled. This becomes a privacy issue for some of the tribes/nations. Have not gotten my DNA back yet here to tell you if any tribal members have tested at ancestry on their own. Another reason why some people may keep their ethnicity private as well. I have also had people freak out on me on my FTDNA matches for my family not being quite white.Some people dont want to hear their family might be of a different ethnicity. You get all kinds of people.