If your question is there a way to shut off the MODEM via some kind of software or signal Cox sends; no. You can use a normal electrical timer to switch it off, then back on at certain times. They are pretty cheap, but if they can plug a modem back in they can just bypass the timer.
Another option would depend on what kind of router you have. Certain ones have the ability to set a wireless schedule that turns the wireless off at certain times. This would not stop the wired internet from working though. Some routers even have special parental controls or scheduled MAC filters that will block only certain computers, or certain websites, and only at certain times. It all gets very complex though, so if your a laymen, you might need some help.
I think the best answer might be the old fashion method. I assume these people are your family or even children? I know when my mom caught me online late at night, she would take something to disable the computer for a week. Usually the mouse ball or power cord. The problem with all these methods though is they create a kind of escalation as you find ways to stop them, and they find ways to get it. Its the age old battle of parenthood. ::grin::