Interesting question, I have read the meaning of the phrase from here, which means "you've failed right now and you are not allowed any benefits that might ordinarily happen on the way to account or atone for it".
But This meaning does not play any role if it is a preposition or not? Because "Do not pass go", is an order, a command. An order does not have true/false value hence, it's not a preposition.
More Example,
1) Give me a glass of water. ==> An order, not a preposition
2) Do not do this work ==> An order, not a preposition etc.