Officially, the Society has never taught that Apostates sin against the Holy Spirit, or that they will be especially singled out for destruction anymore than plain-jane worldly people who die at Armageddon. An Apostate "can" return to the Watchtower organization. All who merit destruction at Armageddon go to Gehenna, according to Watchtower theology, because none who die at Jehovah's hands are worthy of a resurrection. It is common belief among Jehovah's Witnesses, however, that Apostates have "likely" sinned against the Holy Spirit, and will not likely ever repent and return to Jehovah's organization. The fact that ex-JWs have no interest in returning to the Watchower organization means, in the minds of JWs, that we are guilty of such sin and will forever remain unrepentant until our eternal desatruction.
Thank God the Father, Son, and Holy Spirit that we do not believe that stuff anymore.