|Did you look in Outlook Help?|
"Automatically purge the deleted items
You can configure Microsoft Office Outlook so that when you switch between folders, any items you have marked for deletion are purged from the IMAP mail server.
Click a folder in your IMAP e-mail account.
On the Edit menu, point to Purge Deleted Messages.
Click Purge Options.
The Internet E-mail Settings dialog box opens.
On the General tab, under Purge Options, select the Purge items when switching folders while online check box."