This seems like a viable idea, and I like Varian's suggestion as well.

The kicker is that this could actually be programmed, requiring a little investment in coding then let it run for itself, you would not need any human intervention at all when put in place.

I think it would also be reasonable to see an indicator of some sort on your account page of how many warnings you have against you vs. having to request it from an Admin/Mod. IMHO, it should have the date/time of the infraction, the reason, and the Admin/Mod who initiated it, as well.