First of all, in your main assembly file, search for the following:
Underneath this should be the following lines of code:
move.b ($FFFFF605).w,d0 andi.b #$20,d0; is C button pressed? beq.s loc_3230; if not, branch addq.w #1,($FFFFFFE6).w; increment C button counter
Now, what we wanna do is to comment out those lines of code. In other words, add a semicolon ( ; ) at the beginning of each of those lines. This should be the result:
; move.b ($FFFFF605).w,d0 ; andi.b #$20,d0; is C button pressed? ; beq.s loc_3230; if not, branch ; addq.w #1,($FFFFFFE6).w; increment C button counter
What we've done is told the game to ignore all C button presses at the title screen. C is needed to activate debug mode, so with them not being counted, it is nigh impossible to activate it. You may still be able to activate it with a save state. =P Anyway, congratulations. You've disabled debug mode.