Marc,
Are there commonalities to the items which should be removed from the archive? The only real way to get them out is to do an archive export as an administrator. Otherwise, a feasible option would be to enable the client to remove items from the archive (this is a bad idea btw), have the client use the web search page, restore the items which offend him/her from the archive to the mailbox source, then to delete the contents of the basket from the archive.
IMHO, push this task on to the user if you're comfy giving him/her temporary rights against your vaultstore/archive. If you're not, the only option is to do an export. You can even bounce the entire archive back to the original mailbox and "start fresh" by applying the correct mailbox archival policy.
micah