"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus
Repeal of any Amendment, as in the case of Amendment XXI, must be in the form of a new Amendment. The Regress might pass any bloody thing it likes, and your '2/3rds' applies to that as a Constitutional requirement, but whatever nonsense they pass as a purported Amendment must subsequently be ratified by 3/4th + 1 of the legislatures of the several states.
Even if they did, they wouldn't be able to do away with one of the Bill of Rights. They are part of the Supreme Law and untouchable.