I just got back inside from stuffing branches from my woodpile in the gap under the fence where the ground dips quite a bit to block the neighbor cats' access (When I start my modest pond building project, I plan to use the subsoil for fill dirt because I'm NOT using my good dirt to fill a useless hole!) I'm a bit concerted that if a cat tried to get under in a state of panic, it might get hurt, but this seemed like a good/cheap temporary physical barrier.
Also chased two cats out of the yard -- one was about to savour a nibble of my catnip and the other one looked like it - he - was about to spray a clump of tall grass They went over the back fence -- which is just a post and rail lined with chicken wire -- where the chicken wire's bent down a bit. For the time being, I wedged an unused kiddie pool cover against the corner of fence and shed to block the area.
Yesterday, I put a slate stepping stone "sill" under one of the gates because I've seen them crawl under there with some difficulty (having to flatten themselves on their bellies). So the slate will hopefully close the gap enough to keep them out... and blocked another possible access by a fence post with a couple of rocks.
These cats are used to walking across my back yard on daily basis -- there seems to be a designated feline passageway here. But this has got to stop. Yesterday, I caught another one trying to use the kid's sandbox for nefarious purposes. That's at least 5 cats!
I have 2 kitties of my own. But they're strictly indoor cats, with occasional SUPERVISED outdoor activity. Unfortunately, I have two neighbors that keep "adopting" strays. They do trap them and have them fixed/vaccinated, but then leave them outside. I hear that some of the cats that they feed actually belong to someone else on the next street, but come around for the freebies.
So, do you think systematically blocking their access will be sufficient? I'm seriously tempted to spring for the motion-actived water-shooter.