Sounds like you handled the situation well Cladius. Sorry that the actions of others, who are always going to be present it seems, caused you problems.
Well said Snowy. We do have to be positive representitives of our hobby to make up for the actions of others who hurt the hobby. But I don't think I would try to explain the actions of those that did a criminal act. I would do my best to find out who they are and report that to the land owner to deal with them the way he sees fit.
On the other hand if we're talking about those MD'ers that leave trash and don't fill in their holes then I would try to find out who it is, if possible, and talk to them about the rules of ethics of MD'ing and how their actions hurt the hobby.
To make it short and to the point, we need to deal with each situation and our actions should be dictated by the severity of the offence of others.
Just my slant on it.