If someone scams or attempts to scam outside of the forum, then they should, and often do receive negative trust once it is determined that the person who tried to scam (successfully or not) is the same person. An example of this is when someone scams someone on skype.

I personally think that the negative trust given to girlbtc is more a personal issue than anything else. I am also not so sure that yahoo's actions make him a scammer, although if there were credible evidence that girlbtc was (attempting to) use their forum to scam people, or if you believed that girlbtc is a scammer, then what yahoo did probably would.

I don't think this is a good reason for yahoo to get removed from the default trust network, although it does appear that yahoo is a "do as I say, not as I do" kinda person, however I do believe there are other reasons for him to be removed, that I have posted elsewhere.