Actually you can, you just have to be the department manager. Then click the more button when viewing the ticket and choose Unassign.
It's stated in this post http://osticket.com/forums/showthread.php?t=13729(http://osticket.com/forums/showthread.php?t=13729)
The question I have is it possible to give that permission to everyone or at least all admins?
I'm new to osTicket but figuring this issue out myself and viewing a few other posts it looks like in v 1.6 this was possible? At least by admins?