Hi _techie_
It looks like users are selecting adresses form the OWA autocomplete cache. Unlike Outlook where the autocomplete is stored in an nk2 file, in OWA it is stored in the mailbox. AFAIK there is no way to do a mass clear of the cache.
To delete a single entry from the list using OWA:
Highlight the name in the list using arrow up and arrow down, and press the Delete button on the keyboard).
To delete the entire list using OWA
Log into your mailbox using Outlook Web Access Light
Go to Options - Messaging - Under Email Name Resolution - Click “Clear Most Recent Recipients List”
Hope his helps
G-M