I had the same problem a few years ago. Only there were about 15 cats (the neighbors, 1 was mine). I tried everything that was suggested here and at the hardware store, but nothing worked. So I decided to put up a fence around the flower bed, only the fence was that mesh stuff to keep deer and birds away from trees, bushes and such. Wasnt the prettiest, till I found an almost clear mesh. Was about 3-4 ft high.