Since the days of ASIP, the option to empty the trashes made by users on the server is gone. So here is the terminal command to do it:
% find /Volumes/ -name "Trash Can #*" -print0 | xargs -0 rm -RfUse it with cron to automate the task...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040206044242806