Proserpina
Senior Member
I have used monofilament line as a invisible barrier against deer. You could run some low height wood stakes and whilst stringing the monofilament make sure it is of low strength so that a cat would not get strangled. The encroaching cat would bump into the string and hopefully retreat. I am not too sure what would happen if the animal got entangled/buggered - but then again people in the UK are fond of letting pussycat out vs. keeping them inside. There was a documentary about this UK-based phenomenon a while ago.
http://www.dailymail.co.uk/sciencetech/article-2340714/The-Secret-Life-Cat-What-mischievous-moggies-gets-owners-backs.html
You've never actually watched that, have you?