Hi Dev,
At this time there's not a setting available to disable automatic forwarding for users. It can be done via the Forwarding tab in a user's Account Settings or by copying/rerouting a message to another account with content filtering. However, using custom CSS at the system level, you can hide these options for every domain.
As the System Admin, go to Settings > Personalization > Custom CSS tab. To hide the Forwarding tab when a user goes to their Account Settings, enter the following line of code:
#ctl00_TPH_TabStrip_tabForward { display:none; }
Now, when it comes to content filtering, this is a little tricky. While you can remove a user's ability to select Copy or Reroute Message when using content filtering, there's not a way I can find to completely hide those fields. If content filter is not needed for your users, you can disable the functionality as a whole by editing the domains and disabling Content Filtering on the Features tab. Otherwise, enter these lines of code in your Custom CSS to disable the Copy and Reroute content filtering actions:
#ctl00_MPH_chkActionCopy { display:none; }
#ctl00_MPH_txtActionCopyAddress { display:none; }
#ctl00_MPH_txtActionForward { display:none; }
#ctl00_MPH_txtActionForwardAddress { display:none; }
With those lines of CSS, your content filtering actions page will look like the image below. You can see that users won't be able to select or enter an address for copying or rerouting a message. (The checkbox and text box have been hidden.) Unfortunately, I can't find a way to completely hide the text. Someone with more knowledge of CSS may be able to accomplish that though...
In the meantime, I hope this helps you out. I'll change this thread from a Question to a Proposed Idea though. It'd be more ideal to have a setting to disable a user's ability to automatically forward messages, I'm sure.