Yes, there's an option in the Advanced tab for it...
The other option is to SSH into the box and modify the listen ports for the HTTP server. I did this to change it to a different port from :81, and to change the certificate it uses.
Ok, I've just logged into my smoothwall box. Check the following location:
Then, SSH (with say Putty) into your smoothwall box using port 222 (e.g. edge.site.local:222).
For some reason it loads to the middle of the file, so use Page Up till you reach the top, and then edit the apache config file as you would usually to make it only listen on certain IPs.
You might need to consult your httpd.conf user manual from the apache website to sort it all out. While you're there, feel free to change ports around or point it to use a proper, signed certificate...