People are allowed to vote their stake (or flag with it) however they see fit. There is nobody, even the people who work for steemit, that can stop it. In a decentralized platform you take the good with the bad, if you are willing to accept the upvotes from the community you have to take the flags as well.

In a case like this he would either need to find people willing to upvote the posts to counteract the flags or see if the person flagging him is willing to remove them.