can't keep cats out of attic

We have an open attic area in our garage which we use for storage. the floor to the storage area is actually our bedroom ceiling. We have cats that jump up in the storage area when we open the garage door. It wouldn't be a big deal except that the cats jump around and my husband and I up---bad kitties--- I have put moth balls up there and that sprinkle cat repellent stuff. It's NOT WORKING. Any ideas? Putting screen or fencing up is unrealistic and only a last resort.Help