Code Freeze is a free wordpress plugin to change your wordpress site temporarily into read only state.
Disables adding/editing/deleting new content, media, themes, etc.
Disables installing/activating/deactivating/updating/deleting all plugins
Disables new comments/trackbacks on all content items
Removes the QuickPress widget and notices to upgrade core or plugins
Provides notice to dashboard users that any changes will be lost
Provides generally “read only” access to the dashboard