Use the Lock Space Plugin to prevent non-admin access to your space. When the space is locked, guests are shown a rejection screen and an optional link. Admin users can enter as usual.
Install the Plugin
Search for "Lock the Space" in the Plugin Marketplace, add and Install.
Access the "Lock Space" plugin through the File Menu dropdown.
Adjust the Lockout Settings as preferred. The settings and output are displayed below. You can preview your lockout screen by selecting "Preview"
Title: Main Title Text
Text: Longer informational text. Inform the user on the event, the space, and when to come back.
Button Title: Text displayed on the Button
Redirect URL: Button URL
Preview: Preview the Lock Space Graphic
Enable: Enable Space Lockout