I am so sorry that you are having this problem. Have you reported it to the leader of the group? I am sure that if this person is a problem then they have caused problems to others and maybe somewhere complained?
I have to agree that responding is a bad idea. Responding will only get this person to talk more and cause more problems.
If you have found that no group mods can help then maybe you might have to consider moving to a new group. Your own mental goodness is most important in sharing in these environments.

Jessica