Have you checked to see if OWA is disabled for those users? Here's a page that can hellp with that showing you how to disable/enable OWA per user or per server.
http://www.msexchange.org/tutorials/Configuring_and_Using_OWA_in_Exchange_2000_Server.htmlHere's a copy/paste of part of that page:
Disabling Outlook Web Access for a specific user
As I mentioned previously, OWA is enabled by default for all Exchange users. You can, however, disable OWA for a specific user if you need to. In this way, other users can still use OWA unaffected. To disable OWA for a specific user, follow the steps outlined below:
1. Open Active Directory Users and Computers.
2. If not already select, enable Advanced Features by clicking View | Advanced Features.
3. Expand the nodes until you have located the user of concern.
4. Right-click the user and select Properties.
5. Switch to the Exchange Advanced tab and click the Protocol Settings button.
6. From the Protocols page, click HTTP.
7. From the HTTP Protocols Detail page, deselect the Enable for mailbox setting, as shown in Figure 7.
8. Close out all properties pages and the Active Directory Users and Computers console.