It's a grey area so far as I can tell.
If a ship has EU sanctions on it, and it is in EU waters, then it's fair game. But on the high seas, outside any EEZ, not really. And any navy can assist if they are working with whoever has the EEZ.
Same with drug ships I think. When they are seized, it's usually inside an EEZ, and is done in corporation with the nation whose EEZ it is.
This is the issue with the US and it seizing ships/ blowing up speed boats. They do it based on their own laws and sanctions, so it's not really legal.