In 1.8 it was possible to cancel the PlayerFishEvent to block an entity from getting hooked. This does not seem to work in 1.9.

As an example Citizens had to implement this workaround:
https://github.com/CitizensDev/Citizens2/commit/ec917e1fd673b5288447ce1c00d60e56f5a18a8b
Since that workaround depends on NMS it is not feasible for all developers.

Immediately using Entity#remove() on the FishHook does not seem to help. Waiting one tick and then using Entity#remove() does not seem to help either and causes a broken hook experience where you can repeatedly click with your fishing rod and drag the entity around even without a visual hook or line to the entity.