Re: Amend this!
An amendment cannot be entirely repealed. There's no provision for that. Once it's on, it stays on. The 18th was not repealed - it is just countered by a later amendment, the 21st, which states that the 18th is repealed. The 18th is now a vestigial growth on the constitution - a no-longer-active bit of text, legally irrelevant, but stuck there as an eternal reminder that the US tried to ban alcohol and the attempt did not turn out very well.
It's worth noting that when the time came to ban all manner of other recreational drugs, there was no need for an amendment to the constitution, as by that time the supreme court had ruled that such affairs were within the jurisdiction of the federal government under the commerce clause - even if an individual trade was not inter-state, the economic ties of supply and demand mean that all commerce is inter-state commerce, even if indirectly.