From the dashboard, open the workspace card's actions menu (the ... button in the top right of the card) and choose Delete Workspace. This removes the workspace and all of its charts, milestones, and calendars from NotionStats.
To revoke access on Notion's side as well, go to your Notion settings → My connections and remove Stats. After that, NotionStats will no longer be able to read any pages or databases from that Notion account.