The negative trust was introduced to report and flag scammers to prevent other people from falling into the same trap. It is kind of like a rating system to separate the honest users from the scammers. If you were scammed and you could prove it, then you can flag these scammers with a negative trust.

The opposite is also true, because you want to highlight the trustworthy users on the forum with a positive rating.

Unfortunately a lot of people are misusing the trust system to punish users for personal vendettas or if they have received some negative comments from them. This influence people's behaviour, because they are hesitant to post something that might offend other users.

Just ask this person politely to change their mind and try to resolve this situation without further conflict.