There are two types of national park rangers - there are those that interact on a regular basis with the public, and quite frankly may be overwhelmed in those areas. The second type are sworn federal law enforcement agents. The public doesn't necessarily see them too often, but their primary duty is law enforcement.

Ideally, if you told one of the first rangers, they'd call the second kind. Might depend on how busy they were.

Long story short, I'd call either the law enforcement division of that park, or I'd call a game warden.