You're right: that power of retraction can be negated by others with greater power. That's no different from saying that a person has a right to free movement and another person has a right to the sanctity of his property: where person B's property is not made free for others to travel through it, person A's right to free movement is null and void. Thus, a poster's right to retract his statements on PM exists only insofar as the site admins choose to respect his wishes in the matter. Person A has the right of free action as long as it does not infringe on person B's right to be free from force in regards to his person or his property.

Thanks for not decapitating my observations. I do recognize the distinction between a disavowal and a deletion, and I agree that the distinction is significant. As we're discussing a retraction of the sort that involves more than simple disavowal, however, I don't think our statements in regards to deletion are in any way devalued by failing to discuss at length other forms of retraction.

I recommend the American Heritage Dictionary of the English Language as a source of precise and accurate definitions, by the way. The OED can at times be too vague, and too inclusive or incomplete, in its definitions. See the American Heritage definition of retract for a treatment of the term that makes clearer reference to its uses.