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The superuser or tenant admin can disable a user. Disabling a user prevents that user from logging into the form server. If the user was had the role frevvo.designer (aka. was a designer user) the users' forms will become inaccessible to other users. Even if the forms were public users will get the error "This resource belongs to a disabled user". To disable a designer user but keep the form accessible, change the user's role from frevvo.designer to frevvo.readonly and set the Enabled dropdown back to true. In this case when the user tries to login they will get the error "Login is currently disabled".
To To disable a user click the 
icon to the right of the user's name in the user list. Set the Enabled dropdown to false. If this user is already logged into the form server they will be able to continue using the form server until they logout. At that point future logins will be blocked with the error "Your account is disabled".