Will mothballs keep cats away? This is a commonly asked question and I believe the answer is yes but please do not use them, WARNING they are toxic.
Mothballs are extremely toxic! They do not discriminate between cats and any other wildlife in your garden including CHILDREN.
Not only are they toxic to animals they will also dissolve into the soil making that toxic, which will kill plants or transfer toxins to vegetables that you will eat!
This is what the United States Environment Agency has to say about Naphthalene, the main ingredient of mothballs
Naphthalene is used in the production of phthalic anhydride; it is also used in mothballs. Acute (short-term) exposure of humans to naphthalene by inhalation, ingestion, and dermal contact is associated with hemolytic anemia, damage to the liver, and neurological damage. Cataracts have also been reported in workers acutely exposed to naphthalene by inhalation and ingestion. Chronic (long-term) exposure of workers and rodents to naphthalene has been reported to cause cataracts and damage to the retina. Hemolytic anemia has been reported in infants born to mothers who “sniffed” and ingested naphthalene (as mothballs) during pregnancy. http://www.epa.gov/ttnatw01/hlthef/naphthal.html
If mothballs do this to human they are lethal to other small animals.
Please try these other Cat repellents first.