First,click on the icon for your MAIN email box. (You have three email boxes, two for outside accounts and the third one collects all emails--that is the one you want to open)
Click the MENU (Blackberry) key
scroll down to OPTIONS and click it
Put the cursor on the "yes" in front of "Download messages automatically"
Change the YES to NO. Save your changes. Your mail should no longer come to your BB.