CLEARING MAILBOX CONTENTS

I have limited users mailboxes to 15MB and prohibit them from sending/receiving mails after they use their space,I have configured mailbox management tab in recepient policy to immediately delete deleted mails from users deleted items folder and scheduled it run always.But when users delete mails to free up space the items still appears in their inboxes and user are prohited from sending mails even after deleting and after the policy clearing the deleted items,i.e no space is being created.
Somebody help.