"Marking" is a practice that has evolved over time. It's a somewhat nebulous concept among the dubs these days -- many of the rank and file not even really knowing what it is!
Basically, someone can be "marked" if they are flagrantly violating Society policies, but not doing anything that would be under the umbrella of disfellowshipping offenses. (Hence the nebulous nature of "marking.")
The elders will meet with the person (supposedly repeatedly) and give them counsel/warning. If they persist there will be a special talk given in which the person is not named, but the offending behavior is talked about. The elder will then say that if someone in the congo sees such behavior they should "mark" that person and not socialize with them other than at the meetings/field service.
There is no "de-marking" talk. When the offensive behavior stops the marking stops. Again, this causes a hell of a lot of confusion sometimes and the practice of "marking" is rarely done.
My old hall "marked" someone once. It was a terrible talk -- the elder said that "someone is exhibiting a Jezebel-like spirit in the hall" blah, blah, blah. After the meeting everyone was like, "what the hell was that all about?"
Simply put: marking doesn't work. The dubs would be smart to get rid of the practice altogether.
Bradley