Never online... that provides the dominant no control and is therefore not a punishment but an abandonment. Seems counter to what should be the dominant's intent. If you're at that point (see next paragraph,) you might as well end the relationship.

In a face-to-face relationship, I would reserve silence as a last resort. And I would make it clear that the submissive has forced me to be silent with her because it is clear that my words (and actions) have had no effect on her behavior. It should also be made clear, (if one follows my perception of this path,) that this is a last resort and if this enforced introspection also fails, she has failed in being capable of being my submissive.

Willfull disobedience will not be tolerated.