Hello Everyone.....please help

I am having an issue with the manifest files.I will explain it in detail so kindly be patient.
In the iw.cfg file i am having a entry as shown below.
[teamsite_templating]
preview_history_limit=5
Hence every user can have atmost 5 manifest files and 5 temporary html files.The preview directory is the default directory. In future as a master user i change the iw.cfg file as shown below.
[teamsite_templating]
preview_history_limit=3
I reset the configuration files for the change to take effect. When i am trying to preview the DCRs,
the preview is returning a blank page.
The reason is the manifest files are not getting deleted and updated with the new ones automatically.However i manually deleted the manifest files and the preview is working fine.If there are 100 users and and the above situation occur then we can't delete the manifest files for all those 100 users.It should delete existing manifest files automatically as i made a change in the iw.cfg file as shown above.
I searched a lot but didn't find the suitable answer.
I request everyone to provide me an appropriate solution.Thanks in advance........