I believe that education is the best way to keep people from harming native plants and animals, and if a species is threatened or endangered, there's usually a reason why. People who visit these parks may not be able to identify the species to know which are common and which aren't, or know what about these species makes the parks unique. Some places try to protect their threatened and endangered species by hiding them away from the public. Others try to get the local visitors interested in being actively involved with species protection. But what's in the parks is there for everyone, since their taxes go to keeping these parks operating. No one should feel that they are so important or that they have ownership of anything in the parks to take it for themself. As the motto goes, "Take only photos, leave only footprints.".
I guess if your cousin and his wife feel like they're so great that they get to take whatever they want so they can "get something for free", I wonder how many other places they do this. Supermarkets, restaurants, clothing stores, drugstores, or other stores? If they get caught (and they eventually will) they might not think so much of getting something for free - what they WILL get is a lot of bad publicity, escpecially if the store files charges and they're arrested and have to go to court. Sure, they'll be able to afford a lawyer who might get them off, but they won't stop newspapers from printing that they were arrested and charged with a crime. And that could affect their employment now and in the future if they're convicted. I have to wonder if they'll think it was worth it then.
I would suggest that if they come to visit again, you keep them entertained away from your local parks.