Can't delete attachments in mac mail
I do not know for the OS X 10.6.8 but on 10.9.5 it works like this

Create new Smart Mailbox



Once it is done finding all emails with attachments

Select all or specific one

Under the Messages tab select "Remove Attachment".

Or

Create a keyboard shortcut for it

System preferences Keyboard - Shortcuts tab

Select App shortcut

Click on (+) sign)

Select Mail in the Application window

Write "Remove Attachments"

Now choose your favorite keyboard combination for it.