Revoking a user's access to your cloud site can be done in two ways. Either via disabling the user's account or by deleting the user's account from your site altogether.
Disabling a user's account
Click the account you wish to disable under Configuration\Account Management\Accounts. The user's account will open in edit mode. Here you can set Account enabled to OFF using the toggle button under the account picture. Once the account is disabled, the user will no longer be able to access the site.
Deleting a user's account
Tick the select box next to the user account(s) you wish to delete under Configuration\Account Management\Accounts. Hit the Delete button in the top right corner to remove the selected accounts. A window will pop up where you can confirm the account deletion. Once you click YES, the selected accounts will no longer be able to access your site. The deleted users will irreversibly lose all their preferences and settings.