I'm not quite sure why you would want to do this as it goes against the design of SMTP. The queue is there to retry messages when there are network or server problems and to store mail ready for local delivery. If anything else is going into your queue then you need to rethink your configuration anyway.
You may be able to fudge the removal of the queue using ACLs and routers, but the best thing to do is probably reduce the bounce timeouts on the messages.
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Yes, this is a problem I see quite often also. The problem is that they are probably spam emails that are attempting to send a bounced message back to the sender because there was no valid address on your server. If a spammer blasts hundreds and hundreds of spams in a dictionary attack toward one or two of your customers, your queue can fill up quickly.
You can run a cron job to clear your queue as often as you like. If you go to www.cpanel.net and do a custom search for "exim queue cron", you should find some help there.