So, who do you contact about that violation?

Here you go: https://www.justice.gov/action-center/report-spam

You can probably guess how effective that is. In practice, unless you can get the FTC or a state attorney general to sue an actual company for you, nothing will ever come of it.

I report spam that I'm unable to unsubscribe from to the FTC via their online page[0]. I've never gotten any response but it does seem to work. I've been dropped from several marketing lists after reporting them to the FTC, and it's unlikely that these marketers decided on their own to remove me from their lists.

[0] https://reportfraud.ftc.gov/#/