In the last decade there have been quite a few bans (cherem) on books published for the Orthodox Jew. Usually these bans are put in place by Israeli Charedi Rabbis. To my knowledge most of these bans are either due to the author not adhering to the banning rabbis' view of Chazal's knowledge of science or due to the stories told about famous rabbinic personalities.
My question is as follows: when a ban is placed on a book does it become "assur" (forbidden) to read/own that book? If so, what is the nature of the issur? (i.e. what is the issur exactly?)