You can add Site-wide topics so they appear on each workspace as an option when posting content. Just go to the Admin tab and click "Topics" on the left under "Site setup." You can also choose whether or not you want the topic to show up in the Site Snapshot for reporting options.
Users will still be able to add topics to a specific page but your site-wide ones will have an asterisk (*) next to them so they are aware.
Topics also appear in the Activity Manager so you can keep track of what type of info your team is posting.