Which RPGMaker are you using? I'd hazard a guess at RM2K3?

I believe that it's possible to do scripting in RM2K3 - you could probably write something that, for the duration of the mini game, takes the Up and Down keys and ignores if they are pressed. You'll have to wait for The Almighty Bluecube, though, I'm afraid. I haven't a clue about scripting in RM2K3.

If it's RMXP you're using, then it should be a lot easier to do, but again, I haven't a clue.