The question to give negative trust to those who are participating in a signature campaign of a ponzi is a very complex one.

On one hand, these people are advertising what is most likely going to turn out to be a scam. In other words they are advertising what is most likely a scam.

On the other hand, (AFAIK), most of the ponzis have not yet scammed as of when they are running their signature campaigns, so they would not be officially be advertising a scam. There are also a lot of other companies that run signature campaigns that are also very sketch (to say the least).

A number of people who are very trusted today even ran Pirate-pass-throughs that most likely did more damage then anyone participating in a signature campaign today would do.

While I would not personally advertise a ponzi myself, I don't think those that do should receive negative trust. I would not consider someone who knowingly advertised a ponzi to be a especially trustworthy, although I would not go as far to consider them to be a scammer either.

You can make a similar statement about those who promote a ponzi by reporting they have received payment. If someone makes the ill-advised decision to invest, say 0.01BTC and they happen to receive 0.02BTC a week later, I don't see anything wrong with them reporting their experience, and to an extent this will prevent a ponzi who has scammed from being able to continue to steal money because of both the lack of recent positive reports and because of the negative reports.

With the above being said, if someone is what appears to be "shilling" for a ponzi, then the fact that they are also advertising such ponzi in their signature should be taken into consideration when deciding to leave them negative trust. I believe that someone who claims that a particular ponzi will be able to sustain 150% returns forever, is someone who is making outright false statements and is untrustworthy.  

Kind Regards
Panthers52