When you share a report with a user, they will automatically gain access to the Reporting (Beta) screen in the Admin panel. If they have no other reporting-related permissions assigned to them, they will only see the Shared with me tab, where they will only be able to download the reports specifically shared with them.
You can prevent individual users from accessing custom reports that have been shared with them by removing them from the Reporting system admin group in the Admin groups screen:
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