I would like to see what my fellow FReepers suggest, but slamming the door on them seems like the most efficient way to deal with them.
Most are just regular folks (a lot of college kids) who need a temporary job and are serving the US in a noble cause, the US census......
That stated, the invasiveness of the questions that they have been given is of concern.
Outside of the mandatory constitutionally mandated questions of number of people and age at the residence, my understanding is that you can refuse to answer all other questions and ask them to leave.........
Now, if you have an Acorn person show up to take the census, then answer the legiimate questions and have fun.....just make sure you video record the entire confrontation because there is a good chance they WILL file a complaint with local law enforcement (regardless of merit) to intimidate you and others.
Video tape them.