I know on every mailer campaign there is a link to unsubscribe.
Is there any way to put a form on a static web page to unsubscribe from a specific mailing list? Something that is separate to the mailer and is online all the time, in case someone deletes the mail or something?
Sure, that's possible. If you go to your list page under Unsubscribe Settings there is code to add an unsubscribe form to your website (just like a subscribe form). Just use that and you'll have a form to unsubscribe people at any time that isn't tied to a campaign.