There is a plant that is sold as being fisliked by cats, I think it is a coleus family member. Lots and lots of white pepper helps, they don't like sneezing when they wee, and plastic rug holders, and wide thick plastic mesh just under the soil make it difficult for them to dig there. It is a cats nature to do this, so making it as unpleasant an experiene as possible should help. Some people swear by the apparatus that squirts water at the cat when it crosses the sensor, which is fine as long as it is only cats that set it off - small children and welcome dogs are unlikely to appreciate it. When you plant bulbs put cuttings of holly, berberis or pyracantha sticks pushed into the ground, very prickly and they won't like that - you can easily remove them later when the bulbs are up. Whatever you do about it, please do not hurt the cats as that is illegal and I'm sure you don't want to go there. i have gardened in the company of cats for many years, and they can go together,f with a bit of thought on your part. Once your planting gets thick they will go away, becasue there will be nowhere to dig.