A subject that is discontinued can still log into the portal. Encounters won't become due for a discontinued subject, thus there won't be any new data to enter. Any existing forms will be available to edit/view based on the interval's portal options.
To prevent a subject from logging into the portal, set the subject's account to inactive. Keep in mind that a subject's account is universal and goes across all projects in the account. Thus, if the subject is enrolled in more than one project all projects will need to be considered.