A single strand of barbed wire about 18" above the ground surrounding your land would 1) not hurt the kitties (#1 priority), 2) not be unsightly as it could easily be covered by plants, 3) should not be effected by HOAs (if applicable), 4) would hurt like HELL if tripped over (trust me on this one), and 5) super easy to install unless you have a major ton of acreage. When I was younger (mid teens), I helped my father and sister string 6 strands of barbed wire across 5 acres of land. Took about a week to do and we didn't do that much bloodletting.
Or you could just train Pepplegoose and Rorschach to be adorable and distract the thieves? Or maybe they could be attack cats?
