First of all, I have to say that I like and enjoy the sparring ground at park (I am MP3).
That being said, I acknowledge it is a player established feature and it is in no way part of the game rules. That means that we, players, are usually invited to follow sparring ground rules, but do not HAVE to do so to be compliant with the game's rules. I like that: if a character has made a purpose of not following the rules, I have no problem with that.
Thus, I don't vote for any enforced rule in the park (through game machanism, or whatever).
It does not mean, I am against retaliating to the people who purposely don't respect the rules. There are a few things I would suggest.
1. Set up a defense that will not harm you when you are chatting at the park. There are a lot of ways for not being too severely impacted by an unwanted attack: use them and share them with people who are in the same situation as you.
2. Organise a counter attack with people who feel like you.
That is the hard part. Because the offender may be very strong (he will be very strong usually, if he last long). I am facing teh case currently with a MP3 who delibarately breaks the rule and attacks in the park, and he has made himself very strong by using tokens on basic creatures. I have not found a way to beat him yet, but if I manage, I promise he won''t find rest fo a big while. And if that means buying tokens myself... well... not sure yet but you get the point.
And if I don't succeed, I will just you tactic #1 for as long as needed.
In summary: organise a resistance group, find the one who can beat the offender, let him kill the offender's defense ritual, then follow up for massive attack with all the people who feel offended, so that the offender gets stuck with no vitality.
Yes, that requires time and organisation. Yes, that is risky... but after all we are in a PVP game... and it's up to us to manage that kind of situation. But IMO don't try to set up some enforcing rules... it's not the purpose of the game and shouldn't be.
My 2 cts of course.