The easiest way to stop whisper campaigns is to stop sharing details about your personal life with those who have no significantly earned your trust. A person can be a damn fine roleplayer, but not be someone you want knowing your life's secrets, you know? What we do creates this amazing feeling of intimacy. Sharing creativity with someone often does. Yet, I think in many ways, it is an illusion. People are people when they are out of character. People who gossip. People who lie. People who don't respect boundaries. Most of us wouldn't tell someone our secrets on the first meeting or even at the fourth. Yet, there is this safety and comfort with doing it online.
I don't say this in effort to 'victim blame' anyone. I say this as someone who has been heart broken and hurt by people I thought I could trust just because we had a super fun time roleplaying together. Just because someone plays your friend in a MUSH doesn't mean they are your actual friend. I think that roleplayers, as a community, have a really difficult time telling the difference in general.