Wow, so many answers for me... 

But I'm a good person, so.... here the magical solution, maybe useful for someone:

public static int DestroyKey(string label, string type)
{
    try
    {
        CryptokiObject obj = findTarget(label, type);
        session.Objects.Destroy(obj);
        return Settings.OK_RESPONSE;
    }
    catch (Exception e)
    {
        return -1;
    }
}