The issue is not about whaling. There are other ways they could oppose it - and I think more effectively. The simplest way would be to use an injection harpon to taint a dead whale before hauled in, making it unusable.
I watched a few of the episodes in the past - and they initially had a black ship with a huge pirate skull and cross bones plus flew a pirate flag. Just for that reason alone, I would think any ship - military or civilian - could blow them out of the water and otherwise open fire on them. Why wouldn't they be declared pirates given they were flying a pirate flag?
I know if we were out in deep blue water and a vessel came towards us as if to ram and flying a pirate flag, we'd decide if we could outrun it and - if no - unless clearly some silly thing drunks were doing - we'd open fire with everything we have (which is quite a bit when we're deep offshore.)
The problem is it is run by some wacked out, desperate old man who isn't very smart determined to be a martyr.