A water pistol??!! My cat is a long-haired Persian - she loves water & would actually think you were 'playing' if you aimed a water pistol at her.
It's true that they know if you are scared of them & will really take the piss.
(A bit like dogs & gangs of teenagers really!)
Stones, sharp objects - just cruel; i wouldn't even do that to the cat-sized rats in this area. Taking a cat to another area in a cage - not nice; for some owners their pets are like their children. If you have issues with someone's pet then tell them - the owners may not know how badly their cat is behaving.
Dogs - cats hate small dogs because small dogs are mean with big egos! My old neighbour had a Yorkie who actually seemed to want to kill my terrified cat. However; my cat scares the life out of another neighbour's large Collie.
Two things are good at scaring off my cat: a) Men's loud voices (although sometimes she sees men as a 'challenge' & actually loves their attention!) and b) The Hairdryer.
The evil combo of noise & fur being blown the wrong way by The Hairdryer sends my cat racing for cover. When she is naughty & hiding under the bed - I only have to plug the hairdryer in & she's out & off down the stairs.
But Siamese cats do not have the same 'fur issues' as Persians - so a hairdryer may not scare them.
I was attacked by another neighbour's Burmese monster cat who decided it had the 'right' to attack my cat & get into my house.
The noise of a metal pan hit with a wooden spoon; combined with full on aggression (staring, shouting with loud, deep voice) meant that Mr Burmese Monster was so scared that he stayed 'Orf moi laaand' for good.
Siamese cats are similar in personality to Burmese cats so the 'loud noise with openly aggressive body language' should work for you too... good luck!