There's a lovely profit waiting for the person who can come up with an answer to this one, I reckon!
Chicken wire stapled over the top of the tubs so that the plants can grow through?

Yes I have get clear bottles fill to the top with water and lye them down on their sides by the plants,this does work. Or car greece smeard around the edges of pots or on walls etc.

Cover them with something the cat does not like, I seem to remember Nosy Potter suggested slate chips. To sharp for their paws. Cats will only mess where there is clear bare earth.

I've covered my planters (that have spring bulbs in them) with small granite chips that I had leaft over from the garden. Seems to be doing the trick.

THANKS FOR ALL THE COMMENTS I WILL TRY EVERYONE I AM AT MY WITS END HE MUST BE DOING IN THE NIGHT BECAUSE I NEVER EVEN SEE HIM IN THE DAY

Hi! I had a similar problem with the cats from down the road pooping on my lawn and in my flowerbeds and I too wanted a non harmful way to stop them. Orange and/or lemon peel. Worked wonders. Just save it when you eat an orange or use a lemon, cut into little bits and scatter over the surface of the pots or where you don't want the cats going. It worked for me and they have never pooped in my garden since! Hope it works for you too ...