You may not have told them about the rules beforehand but you asked them not to dig up those items and they ignored you. It seems that in that person's case, telling them the rules beforehand wouldn't have made much of a difference, since they've disrespected the rules of other towns.
I'd give them negative feedback. There isn't much you can do if they stole your items or if they were generally rude, but hopefully getting negative feedback will make them less likely to trade with other people. If they give you negative feedback, you can always explain the situation to a mod. Did they leave negative feedback to the person who left them negative feedback before?
I don't know if you did this or not and it's not bad if you didn't (since not everyone knows about this and I guess it would be easy to forget) but if someone is wrecking your town, you can flip the wifi switch before they get to the train station and it'll reset, so your town will be back to normal.