Nothing really *NEEDS* to be done to "turn it off". It isn't required to be used for anything. But if you're looking for a quick hack to "disable" it because of some kind of memory restriction, open overland.h and look for:
#define MAX_X 1000
#define MAX_Y 1000
Change both of those to 1 and the map grid will be reduced to 1x1x3 which will take up all of .... 3 bytes. Or it will once you've recompiled
This would be how I recommend it be done just in case one day you decide to implement it. Far easier than wasting your time ripping code out left and right just "because it's there" as some people seem bent on doing.