The easiest way to accomplish that is to have the IT department staff access the wiki from a folder to which they have write access, while everyone else accesses it from a folder to which they do not have write access. If you're a Windows shop with Active Directory then I believe that such arrangements can be readily set up.
Ideally, the users who are not authorised to save changes wouldn't see the editing controls. We can do that by switching in the "Read Only" theme, but it's not particularly convenient for the people saving changes. There is a ticket about enhancing read-only mode:
Best wishes
Jeremy