Update Oct 21st: 

Hi, we are happy to share some new updates to this ticket. If you’re a workspace admin managing users via Atlassian Admin, you can now centrally manage and/or prevent new user-created workspaces. This is now available for Bitbucket Premium users in beta. For further information, please check out this blog (https://bitbucket.org/blog/introducing-shadow-it-controls-for-workspace-admins)