If you only want to teleport people who are in a certain group, you can use the new scoreboard command to create a 'group' of people (using teams), and then teleport only that team with this command (targets nearest player in the group specified)
/tp @p[team=<internal group name>,<other params here>] <x> <y> <z>
To set up your groups, you can create a group like this:
/scoreboard teams add <internal group name> <display name>
And then add people to this group like this:
/scoreboard teams join <internal group name> <user name>
The advantage of this system is also that you can use it actually to separate groups in the actual game. The scoreboard command has a feature that means that the display name of a team will be displayed before the actual username, appearing like this in the chat:
[<display name>] <user name>: ...
More info on this system can be found in the wiki page linked above.
Changing the @p in the first command above to a player name will stop it from working, because parameters that are inside the [ and ] will only work for @p, @a, and @r. So you cannot substitute the first command above to /tp <player name>[r=2] <x> <y> <z>. It just won't work.
If you really want to use the r parameter on a single person, try using the above method that uses teams, but only put one person in the team. Here is a quick way to do this. Type these into the command console:
/scoreboard teams add <internal player name> <external player name>
/scoreboard teams join <internal player name> <player name>
And then in the command block, to teleport that person if they are in a radius of 2, you could use:
/tp @p[team=<internal player name>,r=2] <x> <y> <z>
Alternatively, you could just specify the name of the player in the selector. Following the previous example, if you want to teleport a certain person if they are in a radius of 2, you could use:
/tp @p[name=<player name>,r=2] <x> <y> <z>
Best Answer
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:
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.