Pinecones. Cats doesn't like to step on them.
A motion activated water spray, kitties do not like to get wet.
Angela....I would talk to the owners. Don't want to cause problems but...it is your yard...maybe they need a fence....happened to people I know and they finally built the fence.
Years ago when I had this problem, I found that orange peel helped. Then I found a household spray that was very 'orangy' and just sprayed it around the garden - had to re-spray periodically but it worked. Use it now to keep slugs off my hostas.
Citrus sprays, eucalyptus or clove or rosemary essential oils tend to repel cats, as will large groups of marigolds, lavender, and applications of tea tree oil saturated on cotton balls and placed throughout your plantings.