First of all, I could not locate on my iOS app any section to unban my banned entries.
So, I went to the corresponding section of the web platform.
However, at "http://www.skritter.com/vocab/banned," there is a bug: Checking multiple boxes and clicking "Unban" deletes only one word entry at a time. This is incredibly frustrating, as I have about 165 words to unban :(