There's not many cases of enforcement. Non-response is taken about as seriously as the Robinson–Patman act. I think the Census Bureau is very reliant on people thinking there will be enforcement, however, which is why the materials they send all have a threatening aura. I don't know about the ACS, but for the decennial census I often felt like my job as an enumerator was just to bother people until they'd answer. The case would keep being recycled until we got at least (IIRC) a head count.

They can certainly enforce that you answer the survey. But it's very difficult to enforce a requirement that people answer questions accurately, particularly when they perceive that doing so will expose them to danger.