When you opt your titles in to Kobo Plus, you're required to agree to our Addendum to the Terms of Service. This states that your books should be included in this service for no fewer than ninety days. (Here's a link to the Addendum for your reference).
Once the ninety day period has passed, you'll be able to de-select the opt in button in the Rights and Distribution section of your book. Doing so will remove your book from Kobo Plus.
If you'd like to remove your titles before the 90 day period has expired, please submit a request above, including the ISBNs for the books you'd like to remove. All books are assigned ISBNs on KWL. You can find yours in the eBook Details section of your books product page on our site.