Midi control problems - Mainstage

I have just purchasd a Yamaha YC organ. This, and an addtional keyboard are both connected to my Mini-mac running Mainstage. However, all of the controls of the organ appear to have been automatically assigned by default, to control knobs on some of my virtual synths. Therefore, if I alter any drawbars or knobs on the organ while playing a song, this in turn, changes setting on synths that I have loaded into my set. I suspect there is a simple solution to disable this, but cannot work it out, or find an answer that has already been posted within this forum. Therefore, I'd be extremely grateful if one of you midi-geniuses can tell me how to de-assign all of the knobs (but not the midi notes) on my organ, so they will not interfere with any other VST while using Mainstage. Many thanks, Mark.