[j_s]

I don't know the right phrase for the opposite of 'preaching to the choir', but that's what I think this post here on HN is: the opposite of preaching to the choir.

[wmeredith]

Maybe something like "Fall on deaf ears" or talking to "a brick wall".

[Skroob]

"Preaching to Reddit"

[itmag]

I always used to say "preaching to the satanists" :)