[XXXNoahXXX]

If this is against the rules but you don't want to go to the prof, just go directly to the student in question. Just tell them "I just wanted to talk to you about your HW assignments. You really need to start showing your work out, because I know there are solution manuals out there and you don't want it to look like you just copied it from there. Hopefully this won't be an issue any more."

Or something like that, non-accusatory but they get the message. If it persists after warning, talk with the prof about it, maybe ask him to give a class announcement or send out an e-mail warning of consequences.