How can I keep cats off my garden?

There is a cat in the neighbourhood who is using my garden as a toilet. I have tried putting down commercial cat repellent, but that hasn’t helped.
I don’t mind the cat coming in to my garden, but I do object to it using it as a toilet.
I will be planting my annual flowers out shortly, and don’t want the cat to disturb my plants.
Any suggestions as to what I could do about this?