When you try to delete the Default Workspaces named My Workspace or Team Workspace, the Delete button is grayed out.
Default Workspaces are created when the account or Team is created, it can be renamed currently but not deleted. We have an open feature request with regard to the same on our GitHub Page. Please upvote the request there so that our product team can prioritize and add this to our development roadmap.
If you are unable to delete Workspace other than the default, please reach out to us at firstname.lastname@example.org.