This article explains how to disable the ability to create groups in all Office 365 services that use groups:
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Outlook
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SharePoint
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Yammer
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Microsoft Teams: Both admins and users won't be able to create teams.
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StaffHub: Both admins and managers won't be able to create teams.
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Planner: Users won't be able to create a plan.
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PowerBI
view the full article at https://support.office.com/en-us/article/Manage-who-can-create-Office-365-Groups-4c46c8cb-17d0-44b5-9776-005fced8e618