While there are many interpretations, it should be noted that millstones around the neck is the better of punishment that should be given. Millstones are the easiest penalty which the scripture says should be more than for justice.
""If anyone causes one of these little onesthose who believe in meto stumble, it would be better for them to have a large millstone hung around their neck and to be drowned in the depths of the sea." - Matthew 18:6