To answer your initial question, there shouldn't be a reason why you couldn't delete them. BUT, there is a preferred way of handling that.
What I'd suggest you do is have us send you a trial of Enterprise so you can disable archiving, then delete your archive directory. Then you can swap back to your Professional license once that's done. Just email Sales and they can get that sent over.
The issue, though, is how did it start archiving in the first place? Did you ever have archiving enabled, on the free edition or on an Enterprise trial?
Derek Curtis
COO
SmarterTools Inc.
www.smartertools.com