Only certain amount of threads on sub-forum ie: auto-delete

[kiki0]

Thanks in advance for any help that you are able to provide.

I am running Vbulletin 4.2.0 Patch Level 2

I'd like to be able to set it so that only a certain number of threads are allowed per sub-forum before they auto-delete permanently. I know it's a strange request but our users would like to have this ability so that when they post pictures to the forum they aren't around forever.

So, the way it would work is this:

Sub-Forum is allowed to have 20 threads at any given time. When the 21st thread is created, the oldest thread auto-deletes permanently, thus having only 20 threads back in the sub-forum.

Is there a way I can do this or a mod that is available?

Hopefully I'm explaining myself correctly.