You missed the part "I like cats but.." :-)

You can use a PIR detector in conjunction with an electrovalve + a
sprinkler.. particularly when the group is called aus.gardens.. and you
don't need to clean the mess afterwards or explain to the owner why are you
bringing the cat (leftovers) in a bag :-) :-) :-)

"Jock" :
Or,
Just use the IR detector to actuate a solenoid connected to a full choke
12
gauge shotty's trigger Make sure it's pointing at the detected area.
I reckon the cat would go one way or another.
Jock