It is not so much can you prevent it - it's a feature that you can choose to use or not use. Once you realize it is a programmable setting you can program it as you prefer. Here's how:

On a MX61 (61 Keys) C3 or middle "C" should normally be the third "C" up from the bottom of the keyboard. (Yamaha uses the convention C3 = middle "C")
If you would like to TRANSPOSE your MX61 so that it turns on down one OCTAVE (Octave = -1) you can do so in UTILITY, as follows:

Press [UTILITY]
Select 01: General > press "Enter"
Use the UP/DOWN CURSOR to locate the OCTAVE parameter setting
Use the DATA DIAL or [INC/YES] or [DEC/NO] buttons to set this as you desire.
Press [STORE] to write this preference into your MX61's System settings. (very important)... you will get a message that this is being stored as your apart of UTILITY settings.

This will remain your preference until you either change it, or you re-initialize the MX61 to factory settings.

Hope that helps.