Maybe we should consider if we really want to be meeting with people we can't trust to follow a simple instruction. :-) Otherwise I guess you'll need to implement IRM.

In Outlook 2007, select Calendar
Select Tools\Forms\Design a Form...
Select Meeting Request <Hidden>, then click Open
Select Actions tab
Double-click on the row that says Forward
Un-check the Enabled box and then click OK
Select Form tab, click Run This Form
Fill in the To.., Subject, Location, Start Time, End Time, etc
For a recurring mtg, on the Options tab select Recurrence and edit the settings
Click Send

Outlook 2010 has a bit different menu, but you should be able to figure it out.

This isn't entirely foolproof. I suspect that Blackberrys can still forward a mtg announcement, because they are not tied directly to Outlook.