Look in /var/qmail/queue, here you will find the mail spool folders and all those garbage messages.
There is an app that you can install to remove messages one by one (within the SME server manager) called e-smith-qmHandle-1.0.0-7.noarch.rpm (packaged by Darrell May I think). It can also be used at the command line for batch work:
[root@kryten queue]# qmHandle
qmHandle v1.0.0
by Michele Beltrame
Wrong parameters entered, available ones are:
-l : list message queues
-L : list local message queue
-R : list remote message queue
-s : show some statistics
-vN : display message number N
-dN : delete message number N
-D : delete all messages in the queue (local & remote)
Additional (optional) parameters are:
-c : display colored output
-N : list message numbers only
(to be used either with -l, -L or -R)
You can view/delete multiple message eg -d123 -v456 -d567
Just be careful as once deleted the messages are gone.
I actually do this manually but its tedious; no mail gets lost tho.
Make a point of stopping smtpfront-qmail before you do this and restarting afterwards (or reboot).
HTH