cats seem to be creatures that actively prize their lives (maxims containing
the number 9 notwithstanding): i have always found that a well-placed
airgun pellet works wonders.

before i need to get my coat, what i mean by 'well-placed' is not on the
cat's body. aim near enough to let him/her know that you mean business, but
far enough away to ensure the cat is left unharmed (not counting feline
post-traumatic stress disorder, possibly); about half a metre is
sufficient. the deterrent seems to last a good while, needing a refresher
perhaps once a season.