as much as i love cats i have the same problem but in my back garden, get hold of some curry plant put it pots around the edge of your lawn, cats dont like the smell of it, also lemon balm is good. i got them both and put them around the perimeter of the garden and a month has gone past and no cats mess.

also you can mulch ya flower beds and use weed supresser and broken slate . they like fresh soil so wont do it in the lawn either .you have a way with words lol .