Does anyone know anything about gardening?

See above - works for cats as well as foxes; is legal and doesn't harm the creatures.

In the past I've used a Yukon style bear trap with sharpened teeth and springs from an old Ferrari. I tend to use an open tin of prime John West tuna in the jaws as bait.
If you have a slightly larger garden with stout fences, anti - personnel mines.

Hi, one way is to encourage their cat into your garden this allows the cat to extend its territory, Cats normally shit at the edges of territories, its like a no mans land and they will sometimes share this with other cats, you don't really want your garden to be what effectively is their toilet, as cat territories can sometimes overlap you may have to encourage another neighbouring cat so you get two for the price of one, BTW its doesn't work in just one visit cats need to feel comfortable so when you see it on its back a few feet from you then you know you've cracked it. good luck and enjoy their company.