S3 bucket policy preventing writes from FSx Lustre DRA

You want to prevent FSx Lustre DRA to write to the linked S3 bucket. In that case, you can remove the link to the bucket. https://docs.aws.amazon.com/fsx/latest/LustreGuide/delete-linked-dra.html

I won't suggest updating the bucket policy to prevent it. It will work but FSx will keep on trying to write to it which is unnecessary.

Also, based on your updated question, here's a sample bucket policy that you might create when setting up the FSx Lustre. You can remove the permissions that you do not need. In your case, you might remove s3:PutObject and s3:DeleteObject permissions.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "*"
            },
            "Action": [
               ~~ "s3:AbortMultipartUpload",~~
               ~~ "s3:DeleteObject",~~
                ~~"s3:PutObject",~~
                "s3:GetObject",
                "s3:GetBucketAcl",
                "s3:GetBucketNotification",
                "s3:ListBucket",
                ~~"s3:PutBucketNotification"~~
            ],
            "Resource": [
                "arn:aws:s3:::bucket_name",
                "arn:aws:s3:::bucket_name/*"
            ],
            "Condition": {
                "StringLike": {     
                    "aws:PrincipalArn": [
                        "arn:aws:iam::file_system_account_ID:role/aws-service-role/s3.data-source.lustre.fsx.amazonaws.com/AWSServiceRoleForFSxS3Access_fs-*"
                }
            }
        }
    ]
}