I have the same problem,occasionally, with the neighborhood cats. Anywhere I dig in the garden used to get pooped in almost immediately.
It looks silly, but what worked for me was to put a bunch of sticks into the area I want to protect. I place the sticks so that they stick straight up about 3"out of the ground about 4" away from each other. This looks very silly, but does the job.
There are also chemical repellents you could likely pick up at Home Depot/Lowes/Hardware store. Or you can do what I did for our potted plants... use pinecones in the bottom of those planters. They are just cumbersome enough to make the area not inviting. Note, though, that do need a lot of pinecones.
Some might suggest using cayenne pepper on the ground; I'm not a fan. The cat really doesn't know any better.
For a permanent fix, river rock will also work to keep them out. You do, however, have to garden 'around' it.