One of those large "water pistols" is good for encouraging cats to avoid
your garden. A quick quirt every time you sopt them in your garden and they
soon get the message.

Personally, I'd discuss this with your neighbour first though. Hopefully
they'll appreciate that your just protecting your borders (sic) and not a
pathalogical cat hater.

And a quick squirt should suffice - no need to drown poor kitty!

Paul DS.