well, I hate the damn things. I think it's okay to throw shoes, pressure cooker weights, machetes, bricks, cast iron plants pots - anything at them.
My most effective treatments to date (because I'm not quite so cruel as the above would imply ((although I would throw shoes)) - is
absolutely harmless: i have one of those disco balls / mirror balls hung from my washing line. When it catches the sun the dappled moving lights seem to scare the cats away.
quite harmful but not too bad: carpet grippers on the top of fences, gates etc. That puts the ******* off...