You can disable or close submissions on any form by navigating to Configure → Form Behaviour → Form Availability.
From here you can configure when submissions are disabled.
If any of the above options are met, the form’s submissions will be closed.
When the submissions are closed, by default a message is displayed saying “Submissions are closed”. This text message can be changed using a custom translation.
On Pro and Agency plans, you can also create a custom closed submissions page by navigating to Configure → Form Behaviour → Custom Closed Submissions Page and toggling on "Custom Closed Submissions Page"
Editing the custom closed submissions page is just like editing a form (click and type, select text to change headings, insert images, videos, etc).