Keep users post counter

[AsukaValentine]

Hello
Everytime I delete a thread on the forum the post are discounted from the counters of the users that participate in this thread.
Now I need to delete a lot of threads in the forum (around 18.000) and users will lost a lot of messages, ranks and some permissions based on number of post.

It's possible to mantain the number of post for every user (and forum total post and threads will be good too) when you delete threads? How can I do it? I'm sure when I used phpbb some years ago that deleting threads or messages don't affect post count, I can't understand why I can't change this in vbulletin, specially when you can create different usergroups based on post.

Thanks!