How to disable Joomla log in form

If the user is logged in, that's now how the form should show. It should show a logout button and information about the user. If that's what you're seeing when logged in, then something strange is happening on your site. Do you have any caching extensions installed or anything else that may be showing the logged out view of the form when the user is actually logged in?

As for hiding the form when logged in, I'd recommend reading the Joomla documentation on Hiding modules from logged in users which should help you setup Joomla and the SCLogin module properly.

I hope that helps,
Alex