Registration closed text can be set in the campaign dictionary. When registration is disabled this text will show instead of the registration page.
1) Go to campaigns admin panel.
2) Click on the Settings tab and Campaign Dictionary tab under it.
3) Click the "Page Section - Registration Page - Message to Display If Registration is Closed" item in the list.
4) Click the Edit button on the pop-up.
5) Make the necessary changes and click Save button to save the changes.
6) Refresh the site page to see the changes.