C# – How to tell Fluent NHibernate not to map a class property

I have a class that is mapped in fluent nhibernate but I want one of the classes properties to be ignored by the mapping.

With class and mapping below I get this error:

The following types may not be used as proxies:
iMasterengine.Data.Model.Calendar: method get_HasEvents should be virtual

//my class
public class Calendar : IEntity {
    public virtual int Id { get; private set; }
    public virtual string Name { get; set; }
    public virtual string SiteId { get; set; }
    public virtual IList<CalendarEvent> Events { get; set; }
    //ignore this property
    public bool HasEvents { get { return Events.Count > 0; } }
}

//my mapping
public class CalendarMap : ClassMap<Calendar> {
    public CalendarMap() {
        Id(x => x.Id);
        Map(x => x.Name);
        Map(x => x.SiteId);
        HasMany(x => x.Events).Inverse();
        //what do I put here to tell nhibernate
        //to ignore my HasEvents property?
    }
}