[otrs] forbid cc: or multiple to:

our customers do not seem to understand there is no need to cc: other people in tickets, it opens unnecessary tickets if one of them answers, and some of them use our otrs system to talk to each other. I tried to warn them not to proceed like this, now I'm fed up with this behaviour, I would like to bounce "invalid" tickets with a standard message "do not cc: anyone, and the to: field must contain only *one* adress" Any ideas how to implement this ?? have a nice day ! -- Sébastien