halacha - Which beis din can declare items Hefker?

There is a rule that Hefker Beis Din Hefker, that a court can declare items ownerless.

1. What are the requirements for a Beis Din to declare items ownerless?


2. Are there any courts nowadays with such powers, and if not, when was the last time there was such a court?


3. If there are no such courts, does that mean that we can't institute Takanos which will result in a loss of money to one party? For example, as many follow those opinions that there is no such thing as copyright, is it impossible to make a cherem against copying other's music illegaly (not as a chumra, but as a takana)? If one can, why has no one implemented a takana similar to the law here of retroactively mafkiya kiddushin and biya where there is a problem of aguna (let's say where the husband disappeared). This was (to the best of my knowledge) never implemented even when the Vaad Arba Artzos was around and even in the time of the Rishonim. Does this mean that there were no Beis Dins with the power to take money out even then?