If a flying creature loses the flying trait, it can be targeted by any spell which normally can not target flying creatures. At least until it regains the flying trait. Some of these spells, like Tanglevine, would actually prevent it from regaining the flying trait.
However, I think we need to work on some of our game terms. Defending does not cause a creature to lose the Flying trait. Neither does Bashing. However, putting them on Guard or having them become Slammed does cause them to lose the Flying trait. I think these might be what you were actually referring to, but I could be mistaken.
