userassociation is not being deleted if associated object or user is deleted

Internally I am using your AssociationManager class to create associations between user and project objects.
Everything works fine as expected, but on a delete of either an user or an project object, the responding association is not being deleted.
Apart from causing trouble in my application, this behaviour is different if an ordinary association is created by the AssociationManager between
two non-user objects. In that case the associations are deleted as expected.
do you have a fast workaround for that problem?