None of them left because of spam or signature campaigns. They left for disagreeing with some questions related to the forum or the Bitcoin project and start to support other forks. Sirius got tired, SmokeTooMuch, Mike Hearn and Gavin Andresen began to support other projects. Vitalik seems to find a greater refuge using Twitter and Reddit.

gmaxwell still uses the forum.

I'm not disagreeing with the premise. I think a lot of others left because of spam. I just put that in these specific cases, they never said this was the biggest reason.