how do I "close" a registration while ensuring I loose none of the data previously sumbit

I would prefer to be able to set my form to close as of tomorrow Sept 30th at a specific time if possible.  If it can be set to do so ahead of time that would be preferable but if not I still need to learn how to close registration on the form in real time once we are no longer accepting registration.  Also- can a "pop up message" appear for anyone who tries to register after it has closed with something like "registration has now closed"

Please refer to this guide on how to set limits on the form: How-to-Limit-Submissions-in-a-Form

Just scroll down and you'll see the guide about setting limits based on the Date.

Change the warning message to "registration has now closed" and you're all set.