If the swan needs to lay eggs, she will drop them regardless of a nest. The swans will also go somewhere else to nest if their nesting area is displaced. The worst possible scenario would be that the swan becomes egg impacted which could kill her. But again, birds will drop eggs regardless of a nest if they feel the urge.

As far as if this barricading of the nest is illegal, you would need to refer to your state laws regarding Mute Swans as many states want to display and kill Mute Swans to introduce the larger Trumpeter Swans for Trophy Waterfowl hunting purposes. You might want to be careful of who and how you ask this question as state officials may intentionally try to find the swans and kill them.

Conversely, if state laws forbid barricading state waterways or interfering with waterfowl/wildlife, they could be fined. Again, this depends on how Mute Swans are viewed in your state. If you can show that they have barricaded areas for many species of wildlife/waterfowl without mentioning Mute Swans, you might be more successful in getting the barricade removed. The Regal Swan