I'm suddenly reminded of the Eldar Solitaires. Rarest of the rare, these unique members of the Harlequins undergo some unknown process that renders them souless. They don't seem to negate psyker powers like real Blanks do, but the similarities between them and the Exorcists seems to be there. With the Harlequins being the Eldar version of the Grey Knights (their race's greatest weapon against Chaos) the Exorcist and Solitaire parallels seem even more apparent.
Ehm, Solitaires aren't soulless- it's even so that their souls belong to Slaanesh when they die.