According to the following article you have 2 options to unsubscribe:
First option is to put the /sitecore modules/Web/EXM/UnsubscribeFromAll.aspx or /sitecore modules/Web/EXM/Unsubscribe.aspx to your email footer. With this link users can unsubscribe from the newsletter directly from their mailing. Sitecore will change the url for RedirectUrlPage.aspx.
Second option is to create a Sitecore Form that uses the Email Experience Manager Client API or create a custom form with an API controller which unsubscribes the user with the Unsubscribe() api call.
