Let me guide you through that, DP200749.
There are two ways you can terminate an employee:
Here's how to terminate an employee on the Employee Details:
Via the pay run:
Make sure to finalize the payroll before terminating the employee. You'll be unable to run another one once they're marked as terminated.
You can check this article for more information: Terminating an employee in QuickBooks Online Advanced Payroll.
Let me know if you need further assistance with the process.