(edit for typos)
My thoughts on the matter are that I seem to side with Chewett and TTL. (yet also understand the original complaint. It is irritating for some people, understandably)
The park has rules, that are socially developed, and not enforceable by any "game mechanics" or single authority figure.
While I agree that continued random attacks by an individual are sometimes a pain, I personally have defenses set that give me a win about half the time, against random folks attacking me.
Just ask Pothos and some others, when it goes to random def, I want to win in Def sometimes, which is why I have the creatures I have. I do not set a long chain of single trees intentionally. If I am in hte mood to lose a lot, I would change up my defenses. But most days, even if I want losses, I don't want them "Exclusively". I like to make folks consider the rick of attacking me. If they ask, I will set a single tree or two, that's it.
You want to just sit in the park and chat with folks, grab a friend, it is quite easy to kill off your creatures and leave them un-revived for a while, if social chatter is your goal for idling in the park.
If you want to train, grab a friend and train. If you want to do both at the same time, realize that at any time some clown can come at you with the mallet of his creatures and smack you. It may not be "Fair", but it is "possible" with the current rules of MD, and their intentional open-endedness.
You want to retaliate against someone, there are ways. Even the loreroot guards have a weakness (that I havn't found yet, but many people claim is easy once you know it). Find that weakness, and exploit it with your own creature selection.
Or team up and give that person XP. There are simple rituals to do so, if you understand the combat system.
Try to make the Fenth's Press the new hang out of social chatter by getting all your friends together there. Or Windy's Pub that seems quiet lately.
If the "abusive park violators" continue to just troll the park and attack at will after being XP capped, for no "in-game benefit" and increase of skills, then re-address the situation. Until then, use the options at our disposal.
As for MP4, the situation is a bit more complicated, but there are by no means "no options except making ParkPolice to enforce it".
In interested pondering,
-Maebius