I'm not aware of a function for this, although I suspect there may be one. There are a few hacks to accomplish your goal though.
You could point a pop3 client at the account and set it not to leave the mail on the server. Leaving that to run for a few minutes should rip all the mail out of the mailbox.
You could write a short perl script to accomplish the same goal.
You could delete the account and then create a new one by the same name.
Other than those, I dont know.
