[Zope] prevent user aquisition with custom userfolder

hello all some problem with user and access stuff: iv'e written my own user and userfolder classes which work just fine, but i need a special behaviour: if a userfolder is defined in an objectmanger instance only those users defined in this specific userfolder should be valid in the context. the normal behaviour -- as you might now -- is to inherit the users from the parents it seems that this can not be done in the userfolder itself -- of course somewhere in the zope machinery ther must be some aq_parent stuff when authorizing a user to a specific context has anybody got an idea where i can subclass something and override the dependent methods? thanks in advance, bernd