A few years ago I sent mine back with lots of red ink to show my displeasure.
Sometime later a census contractor showed up at my house for follow up. I got irate and sent them on their way.
Weeks later another one (or same one) showed up and was told to get gone.
The third time I answered the door with a shotgun. I kid you not. They left and never returned.
I refused to answer mine during the last census. An agent showed up at my door and I refused to answer any questions. She went to my neighbors and I stood outside on their sidewalk and asked them to not answer any questions about me.
A few years back a census worker showed up over my refusal to fill in the long form. I explained that I regarded the info as being none of their business, good day. As I let go of the door to allow it to close, she made a move to grab the door to continue the conversation. She then caught the look in my eye, and probably realized that I would treat any such attempt as a home invasion and would respond accordingly.