I suggest announcing and consistently enforcing a policy on what should
and should not be posted in public forums like course newsgroups, with
consequences for failing to follow that policy. You could refer to
http://www.student.cs.uwaterloo.ca/~isg/policies/cheating, which is a
School-wide standard for academic integrity.
You'd think they'd know about the electronic version of the first rule
of construction:
Think twice, post once.
Jeff Voskamp