I am not sure why the herem [excommunication] signed by the GRA is universally ignored. It seems to me to valid from the stand point of halacha. I once was in the old city of Jerusalem in a small public library which had in it a book that contained the actual language of the five different herems that were declared. [The one the Gra signed was the second. ] I looked to see if I could gain any clarity, and the best I could come up with was that Rav Nahman would not be under that herem, but other than that, it seemed to be applicable now just as much as it was then.
Besides that in noticed that the laws of herem stem from nedarim/oaths and have that same category as when someone says ''my bread is a karban [sacrifice] to you.'' This being the case, transgressing the herem of the Gra is deUraita--a prohibition of the Torah.
i might mention that the herem was to warn people from what would injure them, not just to find things to forbid.
