How can I remove old attachments that have already been sent out as part of a campaign? I am starting to build up a fairly large library of attachments and I would like to clean them up without having to go into my site provider domain account.

I am afraid that there is no other way

You could create a script which would clean up the attachments' folder on regular basis and call that script from your server cron job, if you don't want to do it manually