In Exchange 2007, how do i prevent a user from exploding the + of a DL

I have a DL call ‘Accounts’ with X users who are allowed to email to that list.
In the TO, when the + is not explode the mail from a restricted user is rejected, which is what I want to do.
BUT when you explode the + and see all the users, then the same restricted user can send the email to the recipients in this DL group.

Is there a way you can disble the + of the list ?

OR
Is there a way I can allow specific users to send an email to the ‘Accounts’ List.?
Thank you.