On your own ship, and depending on jurisdiction (based on flag flown and the UNCLOS) [1]. Nobody allows running out to find a ship doing things one doesn’t like and then ramming or blocking its path.

In any case, what law are the Japanese vessels breaking? They amended their quotas to comply with the ICJ ruling [2].

Ramming things makes for good TV. It is emotionally satisfying. Practically, however, it is ineffective or even detrimental towards the goal of saving whales. (It is very effective at maintaining certain peoples’ livelihoods and drumming up nationalistic support for whaling in Japan.)

[1] https://en.m.wikipedia.org/wiki/Admiralty_law

[2] the article