I've been having a think about this, and I have thought of a method that would be good.
We have a forum post explaining the rules of pranking like Bengals first post up the top, and people reply to it saying whether they would like to be able to be pranked or not. If they decide they don't want to take part anymore they could edit thier post to say they don't want to anymore. To prank someone you should check they are on the list and have said they are ok with it. If not on the list, assume no. In addition, I would make a rule that counterpranks are ok regardless of the players position on the list, so that people can't prank then edit thier post and get no return prank.
Advantages:
This would be fairly secure, very if people connect thier minecraft accounts or whatever that thing is that shows our skins.
It would be updated quickly. No one person would have the responsibility of constantly editing the origional post with an up to date list. Even if they went entierly inactive the system wouldn't break.
Disadvantages:
Not everone uses the forums. I'm sure this could be overlooked though.
The topic could get long and hard to search though.
Any thoughts?