Hello Sandman,
I have looked into registration in Portals and how people register. There is no Portal login table or checkbox that says they can log in. It's all managed simply from the Contacts table. I see you posted yesterday and Ben gave a suggestion to put the PortalUserName on the screen. You would do the same thing here. You can have a screen that only you could access that would manage contact's Portal Username and passwords. Erasing these two values for a contact would not allow them to login. You can also do an import update to the Contacts table. It's trickier and more dangerous, but it can be done.
It looks like there was a way to manage this a couple of years ago, but was taken away in screen updates.
- Greg