To delete a tag from your nation completely, navigate within your control panel to People > Tags. A pop-up screen will appear. You can either click the edit icon next to the tag you want to delete, or click on the words "View all" to see a complete list of all tags in your nation.

If you're viewing the complete list of tags, click the edit icon next to the name of the tag you want to delete. 

On the following page, you'll see a "Delete tag" button. Clicking that button will delete that tag from your nation.