Clearly these people are hardening their hearts, but they are not as they think damning themselves to Hell by this action alone. It is a sin, but it is not unforgivable. Basically it is impossible to blaspheme the Holy Spirit if you don’t believe in him because your ignorant. Blaspheming the Holy Spirit is basically the criteria on which we are judged as a whole, to rebel against what we know is true with full conscience and to exalt ourselves as God. It is based on what is in your soul, it’s final eternal decision. Yes, whether you die in Mortal Sin or not will determine your fate, but this is not up to chance, it is guided by God’s providence, and no one who would ever repent will be damned. Basically this is why those who are damned to Hell will actually want to go there, they will not be repentent even in Hell, and they will understand that justice demands Hell and that God has been completely fair with them.
I just pray that anyone who actually takes part in this realizes that they can be forgiven and accpeted by God and not feel as if they have permanently doomed themselves if they later realize the truth that God does exist. Also I think we should realize that some of this type of behavior has it’s roots in the non-Christian behavior of Christian’s influencing people’s opinion of Christianity.