What, exactly, is this “major distinction?” I fail to see one. Saying that something is a cherem is the same as saying that it is forbidden.*
The Wolf
Three possible points:
1: Saying something is a “Cherem” does make it Assur, similar to a Neder. Placing someone in Cherem for doing something (which may be the case here, I do not know) is not creating an Issur. (which was my point)
2: Along those lines, there is the ability to make something Muttar (such as water) assur to you via a neder (or Cherem). That does not create an Issur D’Rabbanan on the object.
3: Being that the Cheremim of Rabbanu Gershom were not accepted by all of Klal Yisroel (and needed to be accepted in order to apply), it does not have the status of Issur of Chazal.