Certain people (mods for games) use pms to help run games with special rules. There are also those like myself who use braingle as a way to keep in contact with friends and while I often use talk boxes to do so there was a time i did send over 30 pms in a day and none of them were spam. I think that the best way would be if you have negative karma there's a cap and if you've been here less than let's say 2 weeks - 2 months there should be a cap. Or possibly one good karma required to lose the cap? Lookey here, I've got a window sheet!
Okay, personally I think PM's should not be sent much, I communicate with people mostly by chat, I only PM if it is Private or the person is currently offline for those who believe PM's should be posted in large amounts I do not totally disagree. I do see a way the mods could get their games done even with the special twist, they send a message to Jake or an active user Jake assigned, in which the person is allowed to remove the PM limit till the game is over and the PM limit increases with the score like 500 to 40PMs so on and so forth. I'm WYSE, in name and in life application.