Might want to include a DMCA takedown address, so that you don't get them sent to your upstream provider. I regularly submit takedown requests to other sites when I see my interview questions posted there. I'm sure others do as well.
I am sure that if someone is uploading the specific wording of an interview question, then that would potentially be a copyright violation. But if they're just describing the problem in their own words (for instance "I was asked to design a system for selling movie tickets") then there shouldn't be a copyright issue.
If someone rephrased them I really wouldn't have an issue with that, because the phrasing is fairly important to the particular questions I'm asking. (and the doofuses using these sites probably would miss any nuance when rephrasing them anyway)