I'm actually coming around to the idea, somewhat, as I have more time to consider it, but IMHO it's still a bad idea to make the thing indefinite. Instead, what about at least having a date by which the poster can have the staff review lifted, conditional upon them having demonstrated improved behavior? Indefinite seems more like to just drive someone away from the site altogether while giving them a date to show improvement by seems like a better form of encouragement.
It would be reasonable to keep the date private between staff and the poster in question so that the whole forum doesn't start deliberating on whether or not their probation should be extended or not.
Also, consider changing the name from "Dissident" to "Probation" or something. Boris is right, "Dissident" isn't a good name for it. Sounds too cool for something that's supposed to be a punishment.