I know about the international waters, but couldn't the UN make itself useful in this case, like concerning the arctic and antarctic regions, and impose some rules and restrictions?
There is internal waters (harbors, bays, estuaries, channels, etc.
There is territorial waters 12 miles from base line.
Economic zone 200 miles.
Those are within some coastal state jurisdiction.
The rest is open waters, no jurisdiction.
No policeman to charge the fine,
No fleet to run patrols in international waters.
If the coastal state bans whaling, than the vessels flying that flag will not do the whaling.
If the coastal state does not ban whaling, commercial vessels, under that state flag can do the whaling.
The UN and its organisation IMO (International maritime organization) do make some international legislative, mostly under MARPOL convetion (Polution prevention), and SOLAS (safety od life at sea, covention), and ILO (Labor convention) and this is enforced by IMO member flag and coastal states.
And than it is enforced within jurisdiction of coastal state through national coast guard or port state control agencies.
But to enforce this, subject vessel must first come into the harbor, or at least within territorial waters of coastal state, where agencies can do the policing.
If the vessel sails from japan and return to japan full of whales, UN and IMO cannot do anything.
Generally speaking, you can not detain, stop, or inspect ship flying different flag at high seas by some coastguard vessel flying different flag.
Exemptions are in some cases piracy, gun smuggling, human trafficking, but generally there is no permanently assigned international fleet to do police work.
It is organized on specific areas and specific cases.
Like coalition forces in gulf of Aden to prevent and tackle piracy.
I dont see any state assigning some naval destroyers and cruisers to do police work in Antarctic to prevent whaling. This would be highly unusual.
Edit.
Numbers of whales are increasing in Antartica. Fin whales, blue whales, and humpback whales. This means whatever Japanese are doing, it is not massacre of whales, or it is not un-sustaining practice.
So while politically correct world is "disgusted" with this practice after "whale wars" series, greenpeace actions, and national geographic and discovery channel movies, in reality japanese whaling has minimal effect on whale population.