get a sprinkler system laid out around garden, put in motion sensors, so anything on the move in the garden gets a shower, and the cats will go else where, just remember to turn it off before you go in the garden,
we put this in for a client, she has a couple of switches by the back door, to turn it off before she goes out, her garden (features in the open gardens RHS program)was getting destroyed by cats, as she has an extensive walled town garden, with lots of beds, they were uprooting plants, pooing on pea shingle paths,
it took a little fine tuning with the sensors, but you rarely see a cat in there now, and if you do it's moving fast, as the sprinkler heads in each area go off as soon as anything moves, and stop as soon as the movement has stopped.
the monition sensors also after dark put lights on in each area, so when it was first installed you would see a running cat lit up moving at speed across the garden, great burglar deterrent as well.
keep an eye on Lidl as they sometimes do a good Hozelock compatible sprinkler sets, it really didn't cost much to buy and install all the bits,
well worth it if you want a cat free garden, hers runs off rain storage butts, so once installed it cost very little to run.
sprinklers bother cats more than rain as it is coming upwards in a flinging motion.