As you know, a SharePoint site owner has the permission to create subsite by default (or additional users after you have edited out of the box permissions or created a new one).
Well, SharePoint Online administrators now have the ability to completely block sub site creation from the SharePoint Online administration portal.
To do it, connect to your SharePoint Online administration portal (https://<your tenant>-admin.sharepoint.com) and access the Settings blade
Then scroll down to the bottom of the page to access the Classic settings page link
Finally scroll down to the Subsite Creation section to manage subsite creation; you can choose to block creation of modern subsite or all type of subsite (modern and classic)
NOTE this will apply to ALL sites