Unfortunately you can not delete site collection permanently from share point online using UI. It will automatically be deleted after 30 days.
If you don’t want to wait those 30 days, you will have to use the SharePoint Online Management Shell in order to run
Remove-SPODeletedSite cmdlet.
Just open the SharePoint Online Management Shell and run the following commands:
Connect-SPOService -Url
https://tenant-admin.sharepoint.com -credential ravi@email.com
Remove-SPODeletedSite -Identity
https://tenant.sharepoint.com/sites/tobedeleted
Confirm [Y] or [A]
Then its done!