Have you tried creating a policy for these users, as per:
How to block attachments using Policy in BES 5?
Although the RIM doc does have the line "Changing this rule to No does not prevent a user from downloading or viewing native attachments on a BlackBerry device" - I'm not 100% sure what they mean there, I would need to do some testing
http://docs.blackberry.com/en/admin/..._204054_11.jsp