If you have VirtueMart eCommerce in Joomla, then when a user registers as a customer in Virtuemart, an user is created in VirtueMart and in Joomla user system. When you delete a customer from VirtueMart, it will automatically delete the user from the Joomla user system as well.

1. Log into Joomla Administrator
2. Go to "Components -> VirtueMart" from top menu.
3. Go to "Admin -> Users" from left menu.
4. Find the user and click the remove icon. Confirm the delete.
5. Now the user has been deleted from both VirtueMart and Joomla user system.