You can use third party programs like qmail-remove http://www.linuxmagic.com/opensource/qmail/qmail-remove to accomplish queue management in plesk control panel managed qmail
The Plesk control panel itself provides a modified version od qmHandle ( http://qmhandle.sourceforge.net/ )along with it that can be used with ease to manage the queue
To see the statistics of the queue:

/usr/local/psa/admin/bin/mailqueuemng -s
Messages in local queue:  0
Messages [...]