To terminate an employee, go to the Accounting module and under Administration, select User Management. 

Search for the user by using any of the Search criteria at the top of the page and click on the User's name to open their profile.

Change their status, located on the right side of the page, and update it from Active to Inactive.

Then update the Termination Date and add a Termination Reason.

Select Save & Close on the top right of the page.