You did not say what version of Outlook or exchange you are using but here is how I can do it in 2007 outlook and exchange.
You will need to create a custom meeting form. On this form we will disable the forward button
First select tools –> design this form
Then on the Actions tab, double click forward and then remove the check from in front of the Enable checkbox. You will then publish it and set it as the default for appointments. Now when your meeting invitations go out the forward button will not be there for them.
Now if they are creative they still might be able to get around this 🙂 How about just adding a footer or header to the message stating anyone caught inviting anyone by forwarding this message will be immediately fired.
It is a rough economy out there!