When I needed to do this as of February, 2006, here was the support reply. (Read all of this message for safety sake.)
Exact reply from support was:
"You should just be able to click the "X" next to the regular web hosting
of the domain, not next to the domain itself, to disable regular access."
Indeed I was. So, in the control panel, go to Domains...Manage Domains.
Under the "Web Hosting" column beside the domain/subdomain you want to remove http access of, click the X.
WARNING! If you get the message "Whoa, are you sure?!
Are you absolutely certain you want to trash the domain yourdomainnamehere and all of the services beneath it?
This includes email aliases, email addresses, email mailboxes, email messages, email autoresponders, email lists, MySQL hosts, everything!
Be VERY sure you want to do this!"
you have clicked the wrong X beside DOMAIN instead of the one under Web Hosting. Do not proceed.
Here is what mine shows after removing regular web access (port 80).
Under Web Hosting
none [Add Now]
Under Secure Hosting
secure site (Lock symbol) (yourusername) [Edit] [X]
You've likely already gotten a ticket replied to from support, but just in case, this worked for me.