Before a Tab or Channel can be removed, you will need to remove the stories within the channel first in order for the delete button to appear for the Tab or Channel dialog box.
To remove content from Hub navigate to the story you want to remove, then click on the 3 dots in the top right and select archive.
Navigate to platform configuration > structure - https://app.bigtincan.com.au/admin/structure and select the edit/pencil icon for the Tab or Channel you want to remove.