They mean very different things. "Banned for contributing" means that contributing was the reason for his ban. What he is banned from doing is unspecified. "Ban

They mean very different things. "Banned for contributing" means that contributing was the reason for his ban. What he is banned from doing is unspecified. "Banned from contributing" means he did something bad and they won't let him contribute anymore because of it. What he did to get banned is unspecified.