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 .