I've had this happen a lot also, but don't want it turned off. I really like the feature and would hate to see if to away.
The easiest option that I can think of would be to revise the code so that the reply to email is the users email so that they would get their own email and then hopefully realize their error. Putting an option into the CP to enable/disable the option would take care of everyone.
There was been some talk about people wanting more email integration. It would be really cool it the system could be set up to add any responses to the email back into the PM thread.
