Deleting JPA object fails due to foreign key constraints?

Why would the following query fail due to a foreign key constraint? There is no other way for me to delete the associated data that I am aware of.

Query query=em.createQuery("DELETE FROM Person");
query.executeUpdate();
em.getTransaction().commit();

The I believe the offending relationship causing the problem is the activationKey field:

2029 [main] ERROR org.hibernate.util.JDBCExceptionReporter - integrity
constraint violation: foreign key no action; FKCEC6E942485388AB
table: ACTIVATION_KEY

This is what I have now:

@Entity
@Table(name="person")
public class Person implements Comparable<Person> {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="id")
    private long id = 0;

    @ElementCollection
    @Column(name = "activation_key")
    @CollectionTable(name = "activation_key")
    private Set<String> activationKey = new HashSet<String>();
}