You can do that in the first 3 weeks of having an account using our purge function. This is found at the bottom of the Admin page:
This will take you to the customer list page:
Here you can select individual customers to delete. The page also shows how many invoices and tickets are associated with each customer.
Alternately, you can hit the "Purge All" link to delete all data.
Please note that this will permanently delete all customers, invoices, estimates, and tickets and that the process is irreversible.
Your other settings, templates, and customizations will not be reset -- only customer data will be removed.