Thank Dr. Fusco for the instructions; I still don't know why Toyota put the beeper in the Prius...
1. Power on (not neccesarily ready, IG-ON will do).
2. Set Trip/ODO to ODO
3. Power off.
4. Power on to ready (brake on).
5. Within 6 seconds, press and hold ODO for 10 seconds or more.
6. WHILE STILL HOLDING ODO *after* 10 seconds, Shift from P to R, then
back to P. Now release ODO.
7. You'll see in the trip pisplay "b on" or "b off".
8. Press ODO to toggle.
9. Now power off.
The Repair manual page says, however, that if you disconnect the 12v
battery, you may lose this change. Repair manual also isn't clear that
you should be holding down ODO while shifting (or operating seatbelt
if you wish to cancel seatbelt buzzers, driver or passenger, using
similar steps replacing shifting w/ fastening seatbelts).