An employee is leaving the company, can I deactivate his or her user account on the portal?

Yes.

As a user can have authorisations for several companies, you just need to remove their authorisations for each company to deactivate their account. 

To deactivate a user attached to a company, you must be a “Manager" or an "Administrator" for this company.

1. Click on your logo or the account name in the top banner
2. In the menu, click on "User Management"
3. The list of existing users will be displayed
4. Click on the line of the user you wish to deactivate
5. The user's detail page is shown.
6. Remove all the user's authorisations by clicking on the "cross" icon (1)
   
    
7. Then click on the "Save" button. The list of users is displayed, and the user is no longer in it.