To deactivate a user:
Go to admin settings -> click on users -> select the user you want to deactivate -> click on edit icon at the end of user row.
This will open a form:
Click on Deactivate button to deactivate your teammate.
By clicking on deactivate you will terminate the account permanently.
If you need further need any help, just use the chat function on the bottom right of this page, we are there to help you out! :)