Agreed.

Though actually, Eliezer used similar phrasing regarding Richard Loosemore and got downvoted for it (not just by me). Admittedly, "persistent troll" is less extreme than "permanent idiot," but even so, the statement could be phrased to be more useful.

I'd suggest, "We've presented similar arguments to [person] already, and [he or she] remained unconvinced. Ponder carefully before deciding to spend much time arguing with [him or her]."

Not only is it less offensive this way, it does a better job of explaining itself. (Note: the "ponder carefully" section is quoting Eliezer; that part of his post was fine.)