As far as I know, you don't really unsubscribe, just stop using it.  If you want to stop getting e-mails, you can do that.  The questions and answers stay forever, unless you delete them.  You can delete the answers you gave, but not the questions you asked.  If you want a question to go away, you have to ask the Yedda staff to delete it, or edit it into "---" or something.

If you are being presented with questions in your e-mail, and you want that to stop, you can do that.  Deleting your topics of interest is one way.  there might be an option you can select to not receive e-mails, but I never looked for it.

If all else fails, ask feedback, and the staff will answer.