How do I stop an entity from entering a boat?

Could use on tick, get each entity in area 1, if entity iterator is type boat, if entity iterator has nbt passengers = name of event/target entity. Set nbt to null. Might work. The passengers nbt is a bit odd so idk if it's possible through mcreator.

Could also despawn/break the boat if it got into one, on tick if all delta movement = 0, then do what I said above and check for boat, then despawn entity iterator, or set health to 0.

A boat is just an entity that can be mounted and dismounted. Just check if the entity is ever riding another entity, check if that second entity is a boat, and if so use the "make [entity] stop riding" procedure block.