Inknow foil can work on counters but I'd be afraid of the damage it could do to a garden. I wonder if one of those fake owls would deter them at all.
Discussion
Some people have suggested plastic fencing and I think the goal is just to effectively stop the cats from being able to scratch at the ground easily.
I have been debating getting one of those like sonic cat deterrents that are solar powered and they play a noise anytime a cat gets close to scare them away but considering most of these cats are bold feral cats I'm not exactly sure how well that would go