It's true that there are many people who would abuse a feature like this.
How about a list of 'message buttons' that people can select from a small menu after tapping the sign or bulletin board? These messages would say things like: 'thanks so much for all the help', 'your campsite looks awesome!', 'your market box saved me!', and only positive things like that.
Whenever I visit a friends campsite (whether it's to check out their campsite/camper or to shop in their market box), I always give kudos (even when there is no reward) in hopes of that translating to 'thank you, and I love what you did here!'.