The sonic repeller does not  hurt them it emits electronic chirps in a
certain direction like a security light so you can aim it to only cover your
deck or a certain area I use a inside version to prevent the cats from
coming anywhere near a door that leads outside.
I use one outside near the garbage cans to keep the stray dogs and raccoons
out of them   they go a running when they come into range

Yea, nothing better than getting up on a beautiful, sunny morning, stepping
out to get the mail and a breath of fresh air and finding dead birdie on
your step that some gracious cat brought you as a lovely gift ;)  Not to
mention flower beds (or sand boxes) turned into cat boxes...  It's lovely to
sit out in the yard on the deck and maybe have a bbq and enjoy nice weather
outside while you eat your burgers with the smell of random cats all around
you.  It's disgusting.  My cat uses a litter box.  I clean that out so I,
and the rest of the world, don't have to smell kitty stench.
Years ago, when I lived at my parents' place and was about 15, there was
this one stray dog that kept coming in our yard and would sit and bark to no
end at my dog... This little rat dog (I am NOT a huge fan of small dogs,
especially small yappy ones that wander about the neighbourhood, and right
into our yard) would sit in the yard, on the grass and bark at my dog in her
kennel if we weren't outside, or would have the audacity to come in the yard
and bark at my dog chained up when we were even outside.  You'd go after the
dog and it would growl, bear its teeth and bark at you.  My brother claims
it snapped at him, but I don't know.  I finally had enough of this dumb mutt
and took the hose to it.  It eventually stopped coming around and torturing
my dog, who was ALWAYS outside (the good part of giant dogs!)
I was tempted to let my dog off her chain or out of her kennel to roam our
yard and eat this little dog, but my dog would constantly jump the 6' fence
around the yard... 6' tall fence in the back yards are the max allowed in
the city where my parents live...  I was really glad when that dog stopped
coming around... So, yes, I know how frustrating it can be for someone else
to allow their pets to do whatever they want, whenever they want and
wherever they want.  I'd spray my yard in your case.  Maybe even spray the
bird bath to help the birds ;) lol  Not sure if that would work or not...
buuut... lol
Cats are great animals if a lot of irresponsible owners could manage to mind
their own animals and understand that even if the neighbours love cats, not
everyone wants their yard to become someone elses' cat's playground/battle
ground.

Be careful of the squirrel.  I  had one get in my 100+ year old house
and get into a fight with my indoor cats.

They had to get booster rabies shots.