To remove the portal from my board, do I delete portal.php?
Or can I somehow redirect the portal to my index page?
You can try just removing it but I have a feeling there might be PHP errors (not sure though).
You can set up a redirect without changing any files in your host control panel. It's easier to do that than edit files IMO.
I wouldn't, just go to templates, edit the header and remove the portal link, if your site still redirects to the portal page, redirect to index.php
True, removing the link in the header, forgot about that.
You can safely delete portal.php. By default MyBB doesn't ship with any links to the portal (if you have links to the portal, it would be from your custom template).