The approach I've used in the past is to go to the Kobo website, logon to my account and then proceed to My Library
http://www.kobobooks.com/library/library.html
Once there, select the books you DON'T want to download, and then click the DELETE button.
This moves the books to TRASH
http://www.kobobooks.com/library/trash/index.html (from which they WON'T get dropped unless you do a DELETE FOREVER. Material in TRASH will not be synched to your device, and can be recovered by select the books and then the UNDELETE button.