Cats using flowerbeds as litterbox

Does anyone have anything that works to keep cats from using your flowerbeds as a litterbox? I have new neighbors across the street, and I've caught their cats leaving unwelcome "presents" in my front yard beds. I have been unable to catch the neighbors at home, and even if I do, I doubt I'll have much luck convincing them to keep their cats indoors! (In spite of Homeowner's rules that say no outside cats; they're renters and I've fought those battles before. No support from HOA either).

Help!