From experience, physical barriers such as lots of small (say 30cm) sticks in areas where the cat likes to poo and dig. Not having areas of open soil will also stop it but that is not realistic where you have bulbs and bedding.

Pepper or chilli powder scattered on the soil is supposed to discourage them, but from some of the comments that have been posted in the past, it doesn't always work. Holly leaves do (ouch), but are unsightly and painful when you want to weed. If you have an out-of-the-way patch of soil that you can sacrifice to them, it will tend to stop them going elsewhere where you may have precious plants. There has also been talk in the past that a little (male) human urine in the right area is a great deterrent! I can't answer for that one, but I don't think there is a single, surefire way of addressing this very common problem. You are not alone!

They are not keen on citrus smells, try cutting up lemons/oranges and spread around the bed, this may keep them at bay for a while,then maybe grow ground cover plants so they can't dig!

it works with sprigs of holly strewn across your bed. I did that one year and when it came to tidying up later in the year, I forgot all about it and cursed myself not to do it again!! So lots of sticks (that Kildermorie suggested) poking out of bare soil will work which is a far more sensible idea to me!

Can I give you some food for thought. I had this problem a few years back and followed all the usual advice. Sticks in the ground worked best. The problem I then had was that the said cat, took it upon herself to pooh all around the garden. Faced with a dilemma I removed the sticks and she went back to the original place.
So perhaps worth thinking about just where the poor moggie is doing her business and if it's somewhere in a corner out the way maybe it's best left in one spot.

They do prefer nicely dug soil, so you could try to keep a nice soft patch and leave the rest rather compacted.

I've tried sticks and fishing line as a barrier and very dilute Jeyes's Fluid as a chemical barrier which seems to work for a while. Ther former does tend to look unslightly in the garden though. If you know whose cat or cats are using your garden as a latrine you could collect the poo up in a large pot and present them with it. It might shame them into providing, and training the cat to use a litter tray. Having said, I had a neighbour who told me that her cat liked my garden because I had seived out all the stones; says it all. I left speechless.

i had this problem last spring,an old timer told me to plant tagettes at the entry point the cat was coming in to the garden,it worked,lovely blooms but the aroma it gives off might not be to some peoples taste.

I can tell that you are not a cat owner! Even if you give them a litter tray they will still use the garden [or someone elses] now and again.I agree with Scottish, as cats are creatures of habit, keep a small patch of earth somewhere MUCH better than all round the garden.Still, it's all manure you know and plants are not fussy. Just always wear gloves when gardening.

Precisely, Freeasabird. Ours sit around in a circle, watching me with great interest as I weed and dig and plant, then, when I've finished, they move in and add their own little contribution, and seem to think I should be pleased about it.

My cat watches me dig a hole, comes and helps me to dig, then fertilises the hole for me, before slightly covering the hole and then kindly lets me carry on. He has no SHAME! lol.

We've always had trouble with cats before we got our 2, but I just accept it as part of the natural order of things and look at it as extra fertiliser. Mind you. since we got our 2 cats no other cat has dared to do their business in our garden, so maybe that's your answer ~ get a cat of your own! ;-)

Or a Jack Russell - that'd see 'em off!

It's a good thing that WE have more inhibitions than cats isn't it? Not to mention good indoor plumbing!