I assume there is a "leash gutter and pick up after your dog" Law.

If so, and one person were fined, after word got out, ... no more public poop, or on your lawn.

I don't think there is a rule on chair saving. If someone leaves for more than a bathroom break, give them like 30 minutes, if it really irks you, put their belongings in a bag marked "lost and found" put the bag along side the chair you are now sitting in. Let me know how it goes.