I am personally making my own solution for that kind of task. Using REST is giving you a lot of options. First of all you need to define what is a "Unused space" and then build a "script" based on this. Few examples - https://developer.atlassian.com/confdev/confluence-rest-api/confluence-rest-api-examples
If you don't want to play around with "scripts" there is always an option to use a plugin for example Archiving Plugin for Confluence - https://marketplace.atlassian.com/plugins/com.midori.confluence.plugin.archiving
It depends do you want to pay for it or spend some time and build your own solution.
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