Version: (using KDE KDE 3.5.5)
Take a look at the following application : Mailwasher
I'd fancy a similar automated free application to clean out my mailbox remotely.
A Linux trial can be found here :
The KDE bugzilla is not a place for requests for new applications.
Anyway, it seems KMail already can do what Mailwasher does by using POP filters. If there is anything specific missing in KMail, you can open a new wishlist item, but make sure you check that it is not already requested.