Guy I work with got one of these last year. He called them up and gave them the riot act about it not being actually part of the census and not really constitutional. He finally asked her what the penalty of refusing to fill it out was. $100.
He asked if he could just send in a check or if he had to wait till charges were pressed. She said she'd have to get back with him on that.
Far as I know they never did do anything.
I researched it at the time I got one and as far as I was able to determine, they have no prosecuted anyone for not filling out the ACS. I think they don’t want to risk an adverse court decision.