HOW TO OFF BBM ON BLACKBERRY

1) Turn off all notifications, both audible and visible for BlackBerry Messenger so that you are NOT alerted to any messages UNTIL you wish to look at the application. (See your Profiles for notifcations and alerts).

2) Set your BlackBerry Messenger status as Unavailable or Busy.
Open BBM > My Profile, etc.

One or both of those should solve your problem.