Control-click the Trash mailbox icon and choose Erase Deleted Messages from the contextual menu.
or
Wait, and Mail will permanently delete the messges automatically after about a week.
If a week is too long, set your preferences to a shorter period. Choose Mail-Preferences, double click the account name, click Special Mailboxes, set the "Erase deleted messages when" pop-up menu to your preferred interval. You can choose Never, One Day Old, One Week Old, One Month Old, and Quitting Mail (which makes your trashed messages go away every time you quit mail).
Good luck.
(I mean, yeah, what he said. We were typing at the same time. He's just faster. ;-) )