The society has never been clear on this, they only hint at what sinning against the holy spirit might be.
I remember having this discussion with a couple of pioneer sisters, after receiving a QFR asking "how does one sin against the holy spirit?". After reading the article, we couldn't figure out what the answer was. We went to an elder and asked him "How do you sin against the holy spirit?" He pointed to the article and said the answer is right here. We said that we had studied the article but still did not know the answer and maybe he could show us wich paragraph it was in. He said you have to read it for yourself.
I now realize the elder didn't know the answere either, but wouldn't say so. He just kept treating us like children who hadn't done their homework................ what an asshole.