FCS only allows you to deactivate a user rather than delete a user. We do not allow you to delete a user because data is tied to that user that you do not want to lose.
1. Go into Toolbox Admin and select 'User Administration'
2. Select the 'edit' button next to the user you would like to deactivate
3. Unselect the 'Account Enabled' permission to deactivate the user's account and click "Update User" to save the changes.
4. The user's account will be inactive so they can no longer log in.