Every website has at least one Admin user account. This user has the ability to enable or disable the access of other users who can log into your website's Control Center.
If you are the Admin, to remove access for a user click on Account ⇨ Users, then click on the pencil icon next to a username. Select Enable or Disable next to the Site Access option, then click on the Save Changes button.
Once a user has been disabled, they will no longer be able to log into the Control Center for that website. If the user has access to multiple websites, the above steps need to be taken in the Control Center for each of those websites.