I've heard back from our developers on this, I may have been mistaken.
The deletion of user statistics only occurs once per month. So you would need to wait a month since the last time the cleaner had ran in order to see these files purged. This functionality has been confirmed and is working with SmarterMail 13 as designed.
At this time there is not a way to 'Force' the deletion to take place prior to the month being over since the last scan was ran.
I hope this helps clarify.
Technical Support Specialist / Internal IT