cat poop

Damn cats! I'm mostly ok with that way they know that I'm allergic to them, which means they immediately run up to me and rub all over me. I'm mostly ok with them spending alot of time under our porch, or in the garage when it got cold. I'm mostly ok with them hanging out on the porch on a nice sunny afternoon, it's a pretty pleasant place to be. I even let peeing on the mustang cover slide by...but know we have reached the final straw. Pooping all over our yard is just not polite. We don't shoo them away, call the humane society or impinge on they're life in any way. But this is the final straw! One poop I might excuse, but in the course of two hours, I stepped in cat poop twice! I had to spend the rest of the day wearing two different shoes! Anyone know how to keep unwanted cats from your yard? Maybe a big mean dog will do the trick...