If you want to remove the completed Notes in your Gneo/Evernote notebooks, you can search in Evernote to find all of your completed Notes and either delete or move them to an Archive completed Notebook within the Gneo Stack on Evernote. This will create an Archive completed Notebook on Gneo and move all of those tasks into this notebook. For more information on searching in Evernote, please read this article on the Evernote website.
If you move your completed to-dos to a Archive completed Notebook outside of the Gneo Stack, your to-dos will still show in Gneo. Further, if you delete the to-dos in the notebook outside of the Gneo Stack from Gneo or Evernote they will delete from the other even if they are outside of the Gneo Stack, this is to keep your accounts in sync.