If you no longer need your organization in Azure DevOps, you can delete it. If you change your mind within 30 days, you can recover your organization. After 30 days, your organization and data are permanently deleted.
It's not able to do this through Rest API at present. You have to do this in web portal.
There are also some limitation to directly call rest api to delete organization. For example you must first meet the prerequisites:
If your organization uses an Azure subscription to bill purchases, you
must first remove billing from your organization in the Azure portal
before you can delete your organization in Azure DevOps.
In the resulting dialog box, enter the name of the organization, and select Delete.

More details please take a look at this official tutorial: Delete your organization