There is a feature in Copilot for Microsoft 365 called Restricted SharePoint Search that allows you to control what SharePoint content Copilot can access. It essentially disables organization-wide search for Copilot and limits it to a specific set of SharePoint sites you choose.
This feature is currently a Public Preview, so availability might differ depending on your Microsoft 365 plan. You'll need Global admin or SharePoint admin privileges and enabling and managing the allowed list involves using PowerShell cmdlets. Refer to Microsoft's documentation for specific commands:
Also, While Restricted SharePoint Search offers more control, it might be simpler to exclude specific SharePoint sites from search entirely. This would prevent both Copilot and regular search from accessing those sites' content, please see below steps -
1. Go to the SharePoint site settings.
2. Click Search settings.
3. Uncheck Allow this site to appear in search results.
I hope this helps!
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