I think its always bad form to break someone's pile and then ask them
to QSY. I don't know exactly how you would ban it, though. Sounds like
the only solution is "peer pressure."
I think that if someone answers your CQ, then you can ethically move them to
any frequency you can talk them into going to. On the other hand, breaking
a pile and having the station QSY is really upsetting to the other stations
calling in the pile.
I've never seen any of the classier op's do this -- and I have always made
a point of remembering the op's that did.