A couple possibilities:

Add dilute ammonia to the squirt gun. Effective against most any animal. I knew someone years ago that carried one on bike rides in the country to repel chasing dogs.

Get a live trap and bait it. Maybe your city or county will even loan you one if you tell them you have a nuisance raccon to remove. Catch the cat then either take it to the pound where the owners will have to pay to get it back or get the garden hose, turn it on full blast up close at the cat for 15 minutes. then let the cat go, it likely won't come back to your yard.

The even bigger problem with wandering cats (bigger than digging in the garden) is the extraordinary number of song birds that they kill.

rich r