What can be done about a website that allows students to cheat on homework?

If this website is run by user generated content (UGC) similar to youtube, there is little that can be done. If the copyright holder sends the website a notice of infringement and the refuse to take material down, they could potentially have an infringement issue, however, until then, there is likely no liability.

Contact your local public and private school super intendant's offices and let them know about the site. Pass the information on to your state's Department of Education as well.

To me, this sounds like a good new way of learning, rather than cheating. Is it wrong for students to share their knowledge with others online? If a student goes to the trouble to look up what other students have posted, sort through it, read several, make a reasoned decision on which one is correct, and adopts the answer, likely with some modification, I'd say that is learning and people should be happy. Learning does not need to come only from schools or officials teachers. Peer learning and online learning are the wave of the now. It is also important to understand that in real life, as opposed to in school, the important thing to know is how to select information, how to choose who might be expert on a topic. No one can know everything and we are all interdependent. If a student learns the fine art of knowing which answer is likely right because of its source or other telltale signs, I'd say that student is learning things that are very important. In life, we don't know it all. We can learn how to find someone who does have the information we need on a particular topic. If students are learning that skill, more power to them.