Can I do something to prevent the leaves of my citrus tree from being nibbled?

A fence of some kind?

Puree some Cayenne peppers add water put it a spray bottle then spray your plant

Get some Bitter Apple spray from any pet store – non-toxic to the plant and cat and should work like a charm

you are not sure it was your cat, so there is a possibility it might be worms… if in case its the worms, mix milk, sugar, and citrus (can be extracted from the leaves).. then spray it on the leaves, take extra care it wont touch the roots because it will ruin it.