You can mark your customers as inactive so you can delete them.
- Go to Sales.
- Select Customers.
- Click the Tick-box beside the CUSTOMERS column.
- Under Batch actions, select Make inactive.
In addition, here's an article you can read to learn more about how you can delete your customers: Manage your customer list.
Lastly, here's a few articles you can read for ideas in managing your customers and sales transactions: Income and Expenses.
Keep me posted in the comments if you have any other questions. I'll be happy to help.