When custom reports are shared with users, they are made available to those users in the Shared with me tab in you share a report with a user, they will automatically gain access to the Reporting (Beta) screen and can also be downloaded by clicking the link in the share notification on the platformin the admin panel. If they have no other reporting-related permissions, they will only see the Shared with me tab, where they will only be able to download the reports 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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