The following article from Microsoft provides some details on how to restrict who can create an Office 365 Group using PowerShell.
Control who can create Office 365 Groups
Every time a user creates an Office 365 group, a SharePoint site collection is provisioned. Depending on how your organization is managing SharePoint Online, you may wish to restrict who has the ability to create new groups. So something to consider as you develop a SharePoint governance plan for your organization.