the neighbours cats keep digging holes in my garden
Cayenne pepper and other hot peppers. But will have to keep putting down. Could be squirrels too.
put chicken wire down plants will grow up between the holes or lay it on ground around base of bigger stemmed plants buy motion sensor water sprinkler(ask neighbor to pay for this expense) Do you know for sure who's cats they belong to? then...Ask your neighbors to provide( & keep clean) their cats with outdoor litter boxes so they quit going in your yard Don't put spices or stuff in your garden that may burn the cats eyes,paws,ears,nose it's not right they are just doing what they need to trying to find somewhere to go to the potty, it's peoples responsibility when they have cats to clean up after them. If it keeps up call animal control & see what your options are if they will talk to people or provide traps to TNR/spay, neuter,return them if strays or feral
Pine cones. They have sharp little edges and could provide some decorative aspect and they are bio-degradable.
Use some Blood Meal (sold at most gardening & DIY stores) Has to be reapplied after watering or rain but it should work.
Ruta graveolens plants, spray essential oils of Rue, Orange oil, tea tree oil, cayenne pepper in areas of your yard frequented by cats. Call animal control if neighbors fail to contain their animals.