I found a way that worked! I installed the Kobo Desktop program on my computer and synced it with my library. When all books were downloaded to the Kobo Desktop, I deleted them from the library (in the program). The effect was that they were removed from the "I'm reading" list
on the site (and thus won't get synced to the Glo), but remained still in the "Purchased" category.
Thanks for your help! This gave me the right idea.