Vickier - The short answer to your question is that the Witnesses do not believe in Hell; they utterly reject the entire concept of Hell as torturous punishment in the afterlife. In fact, they reject the whole idea of an afterlife in any way, shape or form. The Witnesses believe that the "dead are conscious of nothing;" for the Witnesses, the dead exist in a state of unconscious oblivion. The Witnesses use the Hebrew term Sheol [the grave] to designate the collective condition of dead people. The Witnesses regard dead people as existing in a state of total unawareness - an utter lack of consciousness. Witnesses do believe, however, that the dead will be resurrected and live on a paradise Earth. But, again, the Witnesses reject any notion of a Hell,with its attendant eternal torment.
In regard to the supposed sinning against the holy spirit, perhas somebody with more knowledge than me can answer this question in a better fashion. As far as I know, the idea of sinning against the Holy Spirit is indeterminable in their opinion. That is to say that the Witnesses believe that no one can ever really say for sure whether a person has sinned against the Holy Spirit. As far as I know, they leave that determination up to God.