It depends on the options given on your unsubscribe page.  Install Administrators can change the option for the basic subscribe to be a single campaign or all campaigns through the Opt-out of All Campaigns Site Setting.

You can also have a developer create a custom unsubscribe page.  The most common reason for this is to allow users to manage their participation on lists. See the Marketier Developer Guide for more details.

To manually unsubscribe a user, go to Contacts > Search Contacts and simply search for the user or users as you would normally.  Check or highlight the user(s) in question and right-click.  Selecting Unsubscribe brings up the Unsubscribe Contacts dialog box.  You can choose “Unsubscribe from all current and future emails” or “Unsubscribe from Individual Emails” (if your unsubscribe page is set up to allow this). Select the applicable radio buttons and check boxes and click the “Unsubscribe” button to save your changes. 

Yes. The email address associated with the unsubscribed user will remain unsubscribed when you reimport a list, even with different information in other fields.  However, if a user then uses a different email address, they would need to be unsubscribed again. Also, if they change their mind, you can re-subscribe someone manually. Go to Contacts > Manage subscriptions.  From here you can export a list of all unsubscribed users for any individual campaign or manually re-subscribe an email address to all emails or only particular emails.