Scoreboard system for an invisible maze in Minecraft

You could reset that stat objective for anyone not within that radius. You would use a target selector with the radius minimum argument. I recommend putting the command block in the spawn chunk as it will never unload.

Command block is Repeat Unconditional Always Active with command:

scoreboard players reset @a[x=X,y=Y,z=Z,rm=16] [objective]

Where X, Y, and Z are the center of the maze and [objective] is the name of your stat objective.

The only problem with doing it this way is the sidebar display will blink on and off for the players outside the maze when they walk as the stat will increase and then be reset.

You could also set their score to 0 instead of resetting their score. This would prevent the sidebar display from blinking but players outside the maze would still see the sidebar.