If required take backup of the archives
$ cp -a /var/lib/mailman/archives/private/<listname>/* <backup_directory>
Remove the archives
$ rm -rf <listname>/*
Once it is removed recreate the html archive files
$ mailman/bin/arch <listname>
./arun
Category: mailman
Mailman is free software for managing electronic mail discussion and e-newsletter lists. Mailman is integrated with the web, making it easy for users to manage their accounts and for list owners to administer their lists
http://www.gnu.org/software/mailman/index.html
If you have too many mails/spams tend to pending moderator requests in mailman queue it can be removed by:
$ cd ~mailman
$ bin/discard /var/lib/mailman/data/heldmsg--*
And if the no. of argument list exceeds you can try:
$ find /var/lib/mailman/data -name heldmsg--\* -print | xargs bin/discard
* the path of mailman depends on your implementation anyway.