Are they really committing an unforgivable sin? Ultimately, that is for God to judge.
On one level, to abuse the Holy Spirit, from whom the graces of repentance and forgiveness come, certainly endangers any hope of those graces
On another level, as eternally dangerous as it is, this "challenge" is silly - plain and simple. It is like a child who thinks he can make things go away by just closing his eyes or who thinks he can destroy a mighty castle by stomping his little feet.
The one enthroned in heaven laughs;
the Lord derides them...
Again, only God can judge whether they have truly committed an unforgivable sin, for only God truly knows what is in their uninformed minds and confused hearts - even better than these "rational" ones know themselves.
For our part, we should continue to hope and pray for their salvation - and for our own - by the mercy of God.
And for this hope and this prayer there are no better words than those of our Lord (Luke 23:34):
for they know not what they do.