Any ideas for stopping cats using not all but a particular part of my
garden?

Put down shingle in that part of the garden? Not the small pea-shingle but
decent sized beach shingle. I've got that in half my garden and have no cats
using it as a loo. However, they will probably still walk/lie on it?

Do you want to stop them completely from that area of garden or just stop
them from digging in that area?

Best,

- h