I need to find a way to keep certain users from creating new opportunities, thereby only being able to create them by converting a lead.  We have a sales process that starts with lead creation, and it completely throws off our metrics when an opportunity is created directly rather than through lead conversion.  That being said, one team SHOULD be able to do create directly (even if it's only from the 'clone' button).  SF support said I need custom code for this and I've no clue where to start.  Any help would be appreciated.