The US census is in the process of conducting its regular American Community Survey, which has received some notoriety due to some of its invasive questions, to the point where people think it's a phishing scam. Answering the survey is apparently required by federal law under U.S. Code: Title 13. In particular, you can be fined $100 if you refuse to answer any of the questions.
In practice, it appears that this fine is not regularly enforced, but suppose that someone objects sufficiently to the questions that they would rather pay the fine than answer. However, my understanding is that, in such a case, the census bureau has been known to pursue non-respondents quite aggressively, including frequent phone calls and visits to the home.
In this kind of situation, where the action is illegal (and there is a legally-authorized fine), what are a non-respondents' options in dealing with harassment by census bureau employees. E.g., are census workers allowed to enter a home without permission? Are they required to leave if asked?