Connect with and learn from others in the QuickBooks Community.
You can mark your customers as inactive so you can delete them.
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.