Lillies are not good for kids either. Sonic scarers are a waste of money. There are 15 cats around here, we have three, next door has two, next door to her has three, neighbour out the front has two, neighbour across from us has two plus "visitors" that come around. Despite all these cats we have very little in the way of "disputes" between them, dogs and their owners are the biggest problem around here. One of the neighbours fitted a sonic scarer and all the cats used to go up to it and "chin" it (marking scent) so that got taken down quickly. Water won't work on a lot of cats, two of mine love it and they are a nightmare when I am trying to wash the car. Gravel can help, especially if it's 15-20mm as most cats around here don't even like walking on it.
To try to deter mine from going to the toilet in other people's gardens I built a sandpit litter tray in our garden and it has mostly worked, next doors also use it.
We had a nightmare neighbour move in next to us a few years ago who hates cats (should have done his homework before he bought the place) and thought he would be clever putting up a 6 foot fence to keep them out. A fence is not a barrier to cats and the fact that cats have been going through their garden for decades to get to the fields isn't going to stop them. The previous owner wasn't a problem. This moron has put down highly toxic mothballs to deter all cats which birds ended up picking them up so I threatened to report him after reading the data sheet for the ones he was putting down so he picked them all up. Then he sprayed something down which made everyones cats sneeze and have constantly running noses which they kept licking and the vet threatened to report him to the SSPCA after several of us took our cats up with the same symptoms so that put a stop to that. We are expected to put up with his blasted dogs barking their heads off day and night though, bloody hypocrite!! His latest attempt was to spray weedkiller on any cats that went through his garden and I did report him for that, cats do not appreciate having a shower to wash that crap off. I will have anybody that harms or trys to harm any of mine or my neighbours babies and most cat owners will feel the same way so idiots who try to harm the cats be warned!!
I know it's not nice having cats go to the toilet in other peoples gardens and I apologise for that, hell, I even pick up from neighbours gardens if I see any on their lawns or flower beds. There are a lot of cat owners that are not like me but there is no reason to harm the cat, it's only doing what comes naturally and is marking it's territory, it knows no better. Try to have a quiet word with the owner, maybe they could do what I have done and build a outdoor litter tray for them. It only needs a plank or two of decking and a bag of childrens play sand so it costs very little. Do not harm the cats, or any other animal come to that.