you cant join in signature bounties because of trust issues and theres no way to remove it and mostly bounty campaigns are not accepting members with negative trust ,you can try joining in social media bounties .
he cant join in signature bounties but he can join on social bounties ? thats odd . a bounty consists of signature , social media and many more .
so if a bounty manager decide to neglect neg trusted accounts , he cant still join social media bounties but if a manager allow it then he can join all kinds of bounties . neg trust can also be remove depending on the user that gave it or depending on how severe his violation .
There are still some managers who have disregarded the prohibition rules for neg-trust accounts but most of the neg-trusts from hunters are multi accounts that get caught from their wallet addresses so I think it's also hard for accounts with red trust to join the signature.