To disable a user, click on your 'Settings' tab on the left hand navigation.
In your Settings page, scroll down to the 'User Management' section.
PLEASE NOTE: You must be a master user to delete or disable a user.
Click on the three dots on the far right of the user you wish to either delete or disable, then select the disable or delete option.
Please note: If you disable a user, you can reenable them at a later date. However, If you delete the user they will no longer appear in the User Management section and will not have the option to reenable them.