My OH recommends razor blades

but we found that sprigs of holly scattered round worked fairly well but some cats just dont seem to get the idea! Orange peel had some success too but had to keep being reapplied. A neighbour swears by the ultrasonic thingy but they are fairly expensive!
Big Al, good luck with the dog! I had a neighbour whose
dog used to be let out and used our garden as its toilet

For several months I tried politely mentioning it to the neighbour till I lost my cool one day, scooped up the offending (warm) poo and took it to the neighbours house and gave it to her. The dog never came round again! Maybe take the cat poos to the neighbour??
edit; cat pepper never worked for us but maybe I was using the cheap stuff!