Use a portal filter or relationship on the portal that includes matching to a value (or absence of a value ) in that field.
With a portal filter (FileMaker 11 Only), you can simply use IsEmpty ( portalTable::CheckboxField )
With older versions, you'd need to play games with the relationship to get the same results.
Please note that the change in values won't automatically remove the record from another user's portal the instant the value is changed. The other user won't see the record disappear until their screen is refreshed.