I have 3 cats (currently) of my own and have ALWAYS maintained a litter tray indoors for their use.

This minimises the outdoor use of neighbours' toilet facilities - although I'm not dumb enough to assume they never have a call of nature when doing their rounds. It also enables me to keep a check on gut-borne parasites and urinary tract infections and treat at the earliest opportunity.

The most likely form of marking will be by the boys - who neutered or not - will spray to mark their territory!

My best advice would be to get that 'scaredy cat' plant or some of the other suggestions mentioned above.

The only 'shooting' I would condone would be to have a number of WUL bottles full of water and handy about the garden to soak the boogers should they try and soil your territory. One of those 'soaker' guns would be fantastic! This is a humane method but will render the offending cats quite crestfallen and somewhat damp!