NHibernate cascade settings for Roles based authentication

Question

My object model

Person
_______
public IList<Role> Roles { get; set; }
public IList<Group> Groups { get; set; }

Group
_____
public IList<Role> Roles { get; set; }

Role
____
public string Role { get; set; }

So pretty much a standard roles based model, a User can exist in zero to many groups which have roles assigned to them, and can exist in zero to many user specific roles.

How should cascading be setup on this that deleting a person does not delete all of the Roles or Groups, that deleting a Group doesn't delete all people in that group (same for Role, deleting a role doesn't delete all groups or people in it).