how to auto delete mail forwarded from user's mailbox

I have set up forwarding from within the Usermin Webmail mailbox (so the mail hits a Virtualmin mailbox, and gets filtered by Spamassassin, before forwarding). Now I want the forwarded messages to be automatically deleted from the inbox, so the inbox does not get full. How can that be done? Obviously they need to be permanently deleted, not just moved to a deleted mail folder.