Sample IAM policy to restrict access to code commit branch

gistfile1.txt

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Deny",
            "Action": [
                "codecommit:GitPush",
                "codecommit:DeleteBranch",
                "codecommit:PutFile",
                "codecommit:MergePullRequestByFastForward"
            ],
            "Resource": "arn:aws:codecommit:us-east-2:xxxxxxx:reponame",
            "Condition": {
                "StringEqualsIfExists": {
                    "codecommit:References": [
                        "refs/heads/master"   
                    ]
                },
                "Null": {
                    "codecommit:References": false
                }
            }
        }
    ]
}