Hi daz and welcome to the forum! Your account hasn't been deleted because of your original post. For some reason, everything that was posted after March 10 was lost yesterday, which obviously included accounts that have been created after March 10.
About your question:
You can basically do two things to prevent the OB-6 from changing presets on the Pro 2 while both are connected via MIDI. You can either turn off ParamXmit on the OB-6 (press GLOBALS, then program key 5, and select OFF) or turn off MIDI Param Receive on the Pro 2 (press the Global button, scroll to "MIDI Param Receive", and select OFF). I hope that helps.