Okay, through some testing, an update:
After someone gets an infraction for a post, there is a RED CARD down near the edit and quote buttons:
So someone can't get multiple infractions for the same thing.
As a consequence, I'm going to lower the total # of points required for bannings. Those of you with the buttons, just keep an eye out in case someone's already gotten a card for excessive posting (since in this case they'd be able to get an infraction for each one).