It seems moderators are frequently held to different standards by the community, even when exercising none-moderator privileges (such as discussion of opinions).
On some occasions, a user will single out a particular moderator as the target for a variety of blame, insults, or character attacks.
If the user is clearly violating the site rules, it is easy enough to handle this, but what about a user who stays within the bounds of the rules (mostly), yet seeks to undermine the credibility and decisions of that one moderator at every opportunity under the guise of "free speech"? The user criticizes every decision made by that moderator (even if they are supportive when other moderators make similar decisions), tell other users that the moderator can't be trusted, and even repeatedly links external blogs dedicated to how "bad" that one moderator is at doing their job.
So long as that user continues to toe the line and keep their behavior within the bounds of rules, what should be done to handle this situation?