If the post is removed, there could be a notice of that with a reason in addition.
For example
"POST REMOVED FOR: FLAMING. USER WAS TOPIC BANNED FOR THIS POST."
I don't understand the reverse psychology effect argument. Is the concern people will more consciously try to straddle the line just short of infringement? Or is it that by publicizing moderation the perceived "heavy handedness" will cause people to backlash?
I think in the first case, it's still better that there be a line that people can see rather than it being completely arbitrary. One cannot expect compliance to rules with significant discretion in execution unless they have a rough idea how that discretion is utilized. Even in the case the post is removed, it would alert the poster where the line is and give him specifically some insight into where he went over the line. If he becomes problematic trying to straddle he can simply be removed, or the line can be adjusted in light of abuse.
In the second case, the moderation staff has the right to remove anyone for any reason they see fit. Those reacting negatively can certainly be dealt with within the bounds of site authority. But more importantly giving people an explicit understanding of what sorts of behavior is and is not tolerated and how the forum is run will let them make the conscious choice to continue contributing/participating or withdrawing voluntarily.