Apart from building a cage around your garden, you may have to experiment a bit until you find something that works on this particular cat. Some gardeners report success using a variety of deterrents to make the garden less attractive to cats.
One is to scatter thorny branches (such as rose trimmings) over the surface, another is to spread a layer of wire mesh such as chicken wire over or just under the mulch to make digging impossible; another is to use a repellent spray applied to the garden (there are commercial varieties intended to repel cats; or you can make your own concoction with garlic or pepper or both); yet another is to use the “predator urines” packaged for garden use.
Finally, some gardeners report that a pet dog is the most effective deterrent they have tried. You might also check with the local authorities and/or neighborhood association, as in some areas it is illegal to allow pets, including cats, to roam.